Adding comments around serialization handling for clarity

Co-authored-by: David Montague <35119617+dmontagu@users.noreply.github.com>

pydantic/functional_validators.py

@@ -153,6 +153,11 @@ class Model(BaseModel):
     func: core_schema.NoInfoValidatorFunction | core_schema.WithInfoValidatorFunction
 
     def __get_pydantic_core_schema__(self, source_type: Any, handler: _GetCoreSchemaHandler) -> core_schema.CoreSchema:
+        # Note that for some valid uses of PlainValidator, it is not possible to generate a core schema for the
+        # source_type, so calling `handler(source_type)` will error, which prevents us from generating a proper
+        # serialization schema. To work around this for use cases that will not involve serialization, we simply
+        # catch any PydanticSchemaGenerationError that may be raised while attempting to build the serialization schema
+        # and abort any attempts to handle special serialization.
         from pydantic import PydanticSchemaGenerationError
 
         try: