Clarify which hybrid_properties do not have associated expressions
Clarify which hybrid_properties do not have associated expressions

Where a `@hybrid_property` doesn't have an associated `.expression` then it could potentially be called with either `self` or `cls`. This makes it clear in the code, as unintentionally calling with `cls` might not behave as expected.

alphagov/digitalmarketplace-api,alphagov/digitalmarketplace-api,alphagov/digitalmarketplace-api