Correct documentation for @length and @auth traits

[crossleydominic]

Issue:
A couple of instances of incorrect documentation.

Description of changes:

1. Fixes the trait selector for @length documentation to match what is currently defined in the prelude.smithy.
2. Fixes the incorrect example for @auth traits and how effective authentication schemes are determined. This documentation has also been expanded to make it clearer. These examples can be verified correct by applying the following patch:

diff --git a/smithy-model/src/test/java/software/amazon/smithy/model/knowledge/ServiceIndexTest.java b/smithy-model/src/test/java/software/amazon/smithy/model/knowledge/ServiceIndexTest.java
index c36e335eb..a7a50748f 100644
--- a/smithy-model/src/test/java/software/amazon/smithy/model/knowledge/ServiceIndexTest.java
+++ b/smithy-model/src/test/java/software/amazon/smithy/model/knowledge/ServiceIndexTest.java
@@ -102,7 +102,7 @@ public class ServiceIndexTest {
         ServiceIndex serviceIndex = ServiceIndex.of(model);
         Map<ShapeId, Trait> auth = serviceIndex.getEffectiveAuthSchemes(
                 ShapeId.from("smithy.example#ServiceWithNoAuthTrait"),
-                ShapeId.from("smithy.example#OperationWithNoAuthTrait"));
+                ShapeId.from("smithy.example#OperationA"));
 
         assertThat(auth.keySet(), hasSize(3));
         assertThat(auth, hasKey(HttpBasicAuthTrait.ID));
@@ -115,7 +115,7 @@ public class ServiceIndexTest {
         ServiceIndex serviceIndex = ServiceIndex.of(model);
         Map<ShapeId, Trait> auth = serviceIndex.getEffectiveAuthSchemes(
                 ShapeId.from("smithy.example#ServiceWithAuthTrait"),
-                ShapeId.from("smithy.example#OperationWithNoAuthTrait"));
+                ShapeId.from("smithy.example#OperationC"));
 
         assertThat(auth.keySet(), hasSize(2));
         assertThat(auth, hasKey(HttpBasicAuthTrait.ID));
@@ -123,11 +123,22 @@ public class ServiceIndexTest {
     }
 
     @Test
-    public void getsAuthSchemesOfOperationWithAuthTrait() {
+    public void getsAuthSchemesOfOperationWithAuthTraitAndServiceWithAuthTrait() {
         ServiceIndex serviceIndex = ServiceIndex.of(model);
         Map<ShapeId, Trait> auth = serviceIndex.getEffectiveAuthSchemes(
                 ShapeId.from("smithy.example#ServiceWithAuthTrait"),
-                ShapeId.from("smithy.example#OperationWithAuthTrait"));
+                ShapeId.from("smithy.example#OperationD"));
+
+        assertThat(auth.keySet(), hasSize(1));
+        assertThat(auth, hasKey(HttpBearerAuthTrait.ID));
+    }
+
+    @Test
+    public void getsAuthSchemesOfOperationWithAuthTraitAndServiceWithNoAuthTrait() {
+        ServiceIndex serviceIndex = ServiceIndex.of(model);
+        Map<ShapeId, Trait> auth = serviceIndex.getEffectiveAuthSchemes(
+                ShapeId.from("smithy.example#ServiceWithNoAuthTrait"),
+                ShapeId.from("smithy.example#OperationB"));
 
         assertThat(auth.keySet(), hasSize(1));
         assertThat(auth, hasKey(HttpDigestAuthTrait.ID));
diff --git a/smithy-model/src/test/resources/software/amazon/smithy/model/knowledge/service-index-finds-auth-schemes.smithy b/smithy-model/src/test/resources/software/amazon/smithy/model/knowledge/service-index-finds-auth-schemes.smithy
index e95bc72d3..f5e5e236a 100644
--- a/smithy-model/src/test/resources/software/amazon/smithy/model/knowledge/service-index-finds-auth-schemes.smithy
+++ b/smithy-model/src/test/resources/software/amazon/smithy/model/knowledge/service-index-finds-auth-schemes.smithy
@@ -6,11 +6,22 @@ namespace smithy.example
 service ServiceWithNoAuthTrait {
     version: "2020-01-29",
     operations: [
-        OperationWithNoAuthTrait,
-        OperationWithAuthTrait
+        OperationA,
+        OperationB
     ]
 }
 
+// This operation does not have the @auth trait and is bound to a service
+// without the @auth trait. The effective set of authentication schemes it
+// supports are: httpBasicAuth, httpDigestAuth and httpBearerAuth
+operation OperationA {}
+
+// This operation does have the @auth trait and is bound to a service
+// without the @auth trait. The effective set of authentication schemes it
+// supports are: httpDigestAuth.
+@auth([httpDigestAuth])
+operation OperationB {}
+
 @httpBasicAuth
 @httpDigestAuth
 @httpBearerAuth
@@ -18,12 +29,18 @@ service ServiceWithNoAuthTrait {
 service ServiceWithAuthTrait {
     version: "2020-01-29",
     operations: [
-        OperationWithNoAuthTrait,
-        OperationWithAuthTrait
+        OperationC,
+        OperationD
     ]
 }
 
-operation OperationWithNoAuthTrait {}
+// This operation does not have the @auth trait and is bound to a service
+// with the @auth trait. The effective set of authentication schemes it
+// supports are: httpBasicAuth, httpDigestAuth
+operation OperationC {}
 
-@auth([httpDigestAuth])
-operation OperationWithAuthTrait {}
+// This operation not have the @auth trait and is bound to a service
+// with the @auth trait. The effective set of authentication schemes it
+// supports are: httpBearerAuth
+@auth([httpBearerAuth])
+operation OperationD {}

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[mtdowling]

Thanks for submitting this! I just noticed a few typos, but looks great.

[crossleydominic]

All the suggested changes have been made. Thanks.

[JordonPhillips]

Thanks for helping out!