Guides: renaming to active_record_validations_callbacks for consistency and updating links to reflect new path

* 'master' of github.com:lifo/docrails:
  Adds title to activerecord/lib/active_record/associations/*
  Adds title to the rest of the files in activerecord/lib
  Adds title and proper formatting to docs.
  Adds title where needed.
  Adds basic description and title.
  Adds title and minor changes.

Clarify that finder methods return an instance of class ActiveRecord::Relation.

Fixing some minor typos, grammar, accuracy, and spacing issues in Active Record Validations Callbacks guide