Fix doc string of CustomCredential class (#42984)

Author: dargilco

sdk/ai/azure-ai-projects/azure/ai/projects/models/_patch.py

@@ -14,9 +14,9 @@
 class CustomCredential(CustomCredentialGenerated):
     """Custom credential definition.
 
-    :ivar type: The credential type. Required. Custom credential
-    :vartype type: str or ~azure.ai.projects.models.CUSTOM
-    :ivar credential_keys: The credential type. Required.
+    :ivar type: The credential type. Always equals CredentialType.CUSTOM. Required.
+    :vartype type: str or ~azure.ai.projects.models.CredentialType
+    :ivar credential_keys: The secret custom credential keys. Required.
     :vartype credential_keys: dict[str, str]
     """