Apply suggestions from code review

Co-authored-by: Andrius Merkys <andrius.merkys@gmail.com>

optimade.rst

@@ -376,7 +376,7 @@ A few suggestions and mandatory requirements of the OPTIMADE specification are s
 
 ..
 
-    **Note**: A list of database providers acknowledged by the **Open Databases Integration for Materials Design** consortium is maintained externally from this specification and can be retrieved as described in section `Namespace Prefixes`_.
+    **Note**: A list of database and definition providers acknowledged by the **Open Databases Integration for Materials Design** consortium is maintained externally from this specification and can be retrieved as described in section `Namespace Prefixes`_.
     This list is also machine-readable, enabling the automatic discoverability of OPTIMADE API services.
 
 Namespace Prefixes
@@ -425,7 +425,6 @@ They MAY also provide human-readable webpages for their definitions.
 
 Definition-provider-specific fields MAY be fully described at the relevant :endpoint:`/info/<entry_type>` endpoint (see section `Entry Listing Info Endpoints`_), but can also rely on the canonical definitions provided by the definition provider, provided they return an ``$id`` for the field that resolves to the relevant OPTIMADE property definition.
 
-
 URL Encoding
 ------------
 
@@ -3446,7 +3445,7 @@ Custom entry types MUST have all properties described above in section `Properti
 
 - **Requirements/Conventions for properties in custom entry types**:
 
-  - **Support**: Support for any properties in database-provider-specific entry types is fully OPTIONAL.
+  - **Support**: Support for any properties in database-provider-specific or definition-provider-specific entry types is fully OPTIONAL.
   - **Query**: Support for queries on these properties are OPTIONAL.
     If supported, only a subset of the filter features MAY be supported.