Tiny update to callbacks docs [ci skip]

The following was added to the `ActiveJob::Callbacks`,
`ActiveModel::Callbacks` and `AbstractController:Callbacks` docs
in rails#29072:

> NOTE: Calling the same callback multiple times will overwrite
> previous callback definitions.

The comment refers to "calling" callbacks but seems to be about defining
callbacks, as mentioned in the PR description.

In the ActiveJob and AbstractController docs, I believe this will be
misinterpreted as referring to setting callbacks, which, as far as I can
tell, does not have such a restriction.

In the ActiveModel docs, I believe it would be slightly clearer to
replace "calling" with "defining".

Author: nicholasdower

actionpack/lib/abstract_controller/callbacks.rb

@@ -18,9 +18,6 @@ module AbstractController
   # * <tt>skip_after_action</tt>
   # * <tt>skip_around_action</tt>
   # * <tt>skip_before_action</tt>
-  #
-  # NOTE: Calling the same callback multiple times will overwrite previous callback definitions.
-  #
   module Callbacks
     extend ActiveSupport::Concern
 

activejob/lib/active_job/callbacks.rb

@@ -15,9 +15,6 @@ module ActiveJob
   # * <tt>before_perform</tt>
   # * <tt>around_perform</tt>
   # * <tt>after_perform</tt>
-  #
-  # NOTE: Calling the same callback multiple times will overwrite previous callback definitions.
-  #
   module Callbacks
     extend  ActiveSupport::Concern
     include ActiveSupport::Callbacks

activemodel/lib/active_model/callbacks.rb

@@ -60,7 +60,7 @@ module ActiveModel
   # Would only create the +after_create+ and +before_create+ callback methods in
   # your class.
   #
-  # NOTE: Calling the same callback multiple times will overwrite previous callback definitions.
+  # NOTE: Defining the same callback multiple times will overwrite previous callback definitions.
   #
   module Callbacks
     def self.extended(base) # :nodoc: