docs(spdx-utils): Document each main class with a link to the spec

Also slightly reword the existing docs while at it.

Signed-off-by: Sebastian Schuberth <sebastian@doubleopen.org>

utils/spdx/src/main/kotlin/model/SpdxAnnotation.kt

@@ -25,6 +25,7 @@ import org.ossreviewtoolkit.utils.spdx.SpdxConstants
 
 /**
  * An annotation which can relate to [SpdxDocument]s, [SpdxFile]s, or [SpdxPackage]s.
+ * See https://spdx.github.io/spdx-spec/v2.2.2/annotations/.
  */
 data class SpdxAnnotation(
     /**

utils/spdx/src/main/kotlin/model/SpdxChecksum.kt

@@ -20,7 +20,8 @@
 package org.ossreviewtoolkit.utils.spdx.model
 
 /**
- * Specifies checksum value and algorithm.
+ * A checksum for an [SpdxFile].
+ * See https://spdx.github.io/spdx-spec/v2.2.2/file-information/#84-file-checksum-field.
  */
 data class SpdxChecksum(
     /**

utils/spdx/src/main/kotlin/model/SpdxCreationInfo.kt

@@ -24,7 +24,8 @@ import com.fasterxml.jackson.annotation.JsonInclude
 import java.time.Instant
 
 /**
- * Metadata corresponding to a [SpdxDocument].
+ * Creation information for an [SpdxDocument].
+ * See https://spdx.github.io/spdx-spec/v2.2.2/document-creation-information/.
  */
 data class SpdxCreationInfo(
     /**

utils/spdx/src/main/kotlin/model/SpdxExternalDocumentReference.kt

@@ -22,8 +22,8 @@ package org.ossreviewtoolkit.utils.spdx.model
 import org.ossreviewtoolkit.utils.spdx.SpdxConstants
 
 /**
- * [SpdxExternalDocumentReference]s are used by [SpdxDocument]s to list all external documents which are referenced from
- * that particular [SpdxDocument].
+ * A reference from one [SpdxDocument] to another external [SpdxDocument].
+ * See https://spdx.github.io/spdx-spec/v2.2.2/document-creation-information/#66-external-document-references-field.
  */
 data class SpdxExternalDocumentReference(
     /**

utils/spdx/src/main/kotlin/model/SpdxExternalReference.kt

@@ -29,8 +29,8 @@ import com.fasterxml.jackson.databind.annotation.JsonDeserialize
 import com.fasterxml.jackson.databind.deser.std.StdDeserializer
 
 /**
- *  References an external source of additional information, metadata, enumerations, asset identifiers, or downloadable
- *  content believed to be relevant to the Package.
+ *  A reference to an external source of additional information for an [SpdxPackage].
+ *  See https://spdx.github.io/spdx-spec/v2.2.2/package-information/#721-external-reference-field.
  */
 data class SpdxExternalReference(
     /**

utils/spdx/src/main/kotlin/model/SpdxExtractedLicenseInfo.kt

@@ -22,7 +22,8 @@ package org.ossreviewtoolkit.utils.spdx.model
 import com.fasterxml.jackson.annotation.JsonInclude
 
 /**
- * This class holds information about any license not on the SPDX license list.
+ * Information about other licenses that are not found on the SPDX license list.
+ * See https://spdx.github.io/spdx-spec/v2.2.2/other-licensing-information-detected/.
  *
  * Note: The above specification says that several fields are mandatory only if the license is not on the SPDX license
  * list and at the same time says that instances of this class should only be created for licenses not on the SPDX

utils/spdx/src/main/kotlin/model/SpdxFile.kt

@@ -29,7 +29,8 @@ import org.ossreviewtoolkit.utils.spdx.SpdxExpression.Strictness.ALLOW_LICENSERE
 import org.ossreviewtoolkit.utils.spdx.isSpdxExpressionOrNotPresent
 
 /**
- * Provides important metadata about a particular file of a software package.
+ * Information about a file used in an [SpdxDocument].
+ * See https://spdx.github.io/spdx-spec/v2.2.2/file-information/.
  */
 @JsonIgnoreProperties("ranges") // TODO: Implement ranges which is broken in the specification examples.
 data class SpdxFile(

utils/spdx/src/main/kotlin/model/SpdxPackage.kt

@@ -27,7 +27,8 @@ import org.ossreviewtoolkit.utils.spdx.SpdxExpression.Strictness.ALLOW_LICENSERE
 import org.ossreviewtoolkit.utils.spdx.isSpdxExpressionOrNotPresent
 
 /**
- * Describes a software package.
+ * Information about a package used in an [SpdxDocument].
+ * See https://spdx.github.io/spdx-spec/v2.2.2/package-information/.
  */
 data class SpdxPackage(
     /**

utils/spdx/src/main/kotlin/model/SpdxPackageVerificationCode.kt

@@ -22,7 +22,8 @@ package org.ossreviewtoolkit.utils.spdx.model
 import com.fasterxml.jackson.annotation.JsonInclude
 
 /**
- * A unique identifier based on the file contents of a package.
+ * An identifier for an [SpdxPackage] based on its file contents.
+ * See https://spdx.github.io/spdx-spec/v2.2.2/package-information/#79-package-verification-code-field.
  */
 data class SpdxPackageVerificationCode(
     /**

utils/spdx/src/main/kotlin/model/SpdxRelationship.kt

@@ -22,7 +22,8 @@ package org.ossreviewtoolkit.utils.spdx.model
 import com.fasterxml.jackson.annotation.JsonInclude
 
 /**
- * A relationship between two SPDX documents, packages or files.
+ * A relationship between two [SpdxDocument]s, [SpdxPackage]s or [SpdxFile]s.
+ * See https://spdx.github.io/spdx-spec/v2.2.2/relationships-between-SPDX-elements/.
  */
 data class SpdxRelationship(
     /**

utils/spdx/src/main/kotlin/model/SpdxSnippet.kt

@@ -28,8 +28,8 @@ import org.ossreviewtoolkit.utils.spdx.SpdxExpression.Strictness.ALLOW_LICENSERE
 import org.ossreviewtoolkit.utils.spdx.isSpdxExpressionOrNotPresent
 
 /**
- * A Snippet can be used when a file is known to have some content that has been included from another original source.
- * It can be used to denote when a part of a file may have been originally created under another license.
+ * Information about a snippet that has different licenses and / or copyright than the containing file.
+ * See https://spdx.github.io/spdx-spec/v2.2.2/snippet-information/.
  */
 @JsonIgnoreProperties("ranges") // TODO: Implement ranges which is broken in the specification examples.
 data class SpdxSnippet(