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.
Add docs for have_secure_password
Add docs for validate_absence_of
Add docs for validate_acceptance_of
Add docs for validate_confirmation_of
Add docs for validate_numericality_of
Add docs for validate_presence_of
Add docs for validate_uniqueness_of
Add docs for accept_nested_attributes_for