Clarify use of rescue_from [ci skip]

`rescue_from` works for rescuing exceptions in controller actions, but it's not specific to ActionController.

This change updates the docs to clarify the specifics of how `rescue_from` is used and that its use goes beyond controller actions.

Note: I believe the original addition of this documentation was added as part of the move from [`ActionController::Rescue` to `ActiveSupport::Rescuable`](rails@259a7a8#diff-2276a3674b84e1262c94cc36346f9fbd8d00e0a4542bb991a8846c06d86335b1R12)

Author: Alec Clarke

activesupport/lib/active_support/rescuable.rb

@@ -14,12 +14,12 @@ module Rescuable
     end
 
     module ClassMethods
-      # Rescue exceptions raised in controller actions.
+      # Registers exception classes with a handler to be called by <tt>rescue_with_handler</tt>.
       #
       # <tt>rescue_from</tt> receives a series of exception classes or class
-      # names, and a trailing <tt>:with</tt> option with the name of a method
-      # or a Proc object to be called to handle them. Alternatively a block can
-      # be given.
+      # names, and an exception handler specified by a trailing <tt>:with</tt>
+      # option containing the name of a method or a Proc object. Alternatively, a block
+      # can be given as the handler.
       #
       # Handlers that take one argument will be called with the exception, so
       # that the exception can be inspected when dealing with it.