[ci skip] Mention context option of save in documentation

`ActiveRecord::Validations#save` takes `context` option
which can change validation context.
This fact should be mentioned in the documentation.

activerecord/lib/active_record/validations.rb

@@ -40,6 +40,7 @@ module Validations
     include ActiveModel::Validations
 
     # The validation process on save can be skipped by passing <tt>validate: false</tt>.
+    # The validation context can be changed by passing <tt>context: context</tt>.
     # The regular {ActiveRecord::Base#save}[rdoc-ref:Persistence#save] method is replaced
     # with this when the validations module is mixed in, which it is by default.
     def save(options = {})