Document reset_changes since it is part of public API

[ci skip]

activemodel/lib/active_model/dirty.rb

@@ -15,6 +15,8 @@ module ActiveModel
   # * Call <tt>attr_name_will_change!</tt> before each change to the tracked
   #   attribute.
   # * Call <tt>changes_applied</tt> after the changes are persisted.
+  # * Call <tt>reset_changes</tt> when you want to reset the changes
+  #   information.
   #
   # A minimal implementation could be:
   #
@@ -36,6 +38,10 @@ module ActiveModel
   #       # do persistence work
   #       changes_applied
   #     end
+  #
+  #     def reload!
+  #       reset_changes
+  #     end
   #   end
   #
   # A newly instantiated object is unchanged:
@@ -59,6 +65,12 @@ module ActiveModel
   #   person.changed?       # => false
   #   person.name_changed?  # => false
   #
+  # Reset the changes:
+  #
+  #   person.previous_changes # => {"name" => ["Uncle Bob", "Bill"]}
+  #   person.reload
+  #   person.previous_changes # => {}
+  #
   # Assigning the same value leaves the attribute unchanged:
   #
   #   person.name = 'Bill'