Add a note on custom validation contexts.

The documentation on `:on` for validations was inconsistent, and most
only referenced the `:create` and `:update` contexts. I fixed those to
be consistent with the documentation on `AM::Validations.validates`,
which seemed to have the best docs.

[ci skip]
Merge pull request #16409 from justinweiss/update_validation_context_documentation

Docs: Add a note on custom validation contexts. [ci skip]
Clarify possible validations you can use to avoid a NULL boolean value
in the database. Closes #16304 [ci skip]