Document callback deduplication with regards to ActiveRecord::Transaction

/cc rails#51184, rails#51171, rails#40648, rails#48910

Co-authored-by: Rob Jonson <[email protected]>

Author: zzak

activerecord/lib/active_record/transactions.rb

@@ -188,6 +188,27 @@ module Transactions
     # #after_commit is a good spot to put in a hook to clearing a cache since clearing it from
     # within a transaction could trigger the cache to be regenerated before the database is updated.
     #
+    # ==== NOTE: Callbacks are deduplicated per callback by filter.
+    #
+    # Trying to define multiple callbacks with the same filter will result in a single callback being run.
+    #
+    # For example:
+    #
+    #   after_commit :do_something
+    #   after_commit :do_something # only the last one will be called
+    #
+    # This applies to all variations of <tt>after_*_commit</tt> callbacks as well.
+    #
+    #   after_commit :do_something
+    #   after_create_commit :do_something
+    #   after_save_commit :do_something
+    #
+    # It is recommended to use the +on:+ option to specify when the callback should be run.
+    #
+    #   after_commit :do_something, on: [:create, :update]
+    #
+    # This is equivalent to using +after_create_commit+ and +after_update_commit+, but will not be deduplicated.
+    #
     # === Caveats
     #
     # If you're on MySQL, then do not use Data Definition Language (DDL) operations in nested