Add docs for allow_value
Add docs for allow_mass_assignment_of
Add docs for ensure_inclusion_of
Add docs for ensure_exclusion_of
Add docs for ensure_length_of
Experiment: extract allow_mass_assignment_of docs to own file
Revert "Experiment: extract allow_mass_assignment_of docs to own file"

This reverts commit 9f8e85d8a3500bd641962e51954788f9160d4c98.