Improve documentation of Procs as :if / :unless options for callbacks

activesupport/lib/active_support/callbacks.rb

@@ -657,9 +657,17 @@ module ActiveSupport
         # * <tt>:if</tt> - A symbol or an array of symbols, each naming an instance
         #   method or a proc; the callback will be called only when they all return
         #   a true value.
+        #
+        #   If a proc is given, its body is evaluated in the context of the
+        #   current object. It can also optionally accept the current object as
+        #   an argument.
         # * <tt>:unless</tt> - A symbol or an array of symbols, each naming an
         #   instance method or a proc; the callback will be called only when they
         #   all return a false value.
+        #
+        #   If a proc is given, its body is evaluated in the context of the
+        #   current object. It can also optionally accept the current object as
+        #   an argument.
         # * <tt>:prepend</tt> - If +true+, the callback will be prepended to the
         #   existing chain rather than appended.
         def set_callback(name, *filter_list, &block)

guides/source/active_record_callbacks.md

@@ -319,6 +319,14 @@ class Order < ApplicationRecord
 end
 ```
 
+As the proc is evaluated in the context of the object, it is also possible to write this as:
+
+```ruby
+class Order < ApplicationRecord
+  before_save :normalize_card_number, if: Proc.new { paid_with_card? }
+end
+```
+
 ### Multiple Conditions for Callbacks
 
 When writing conditional callbacks, it is possible to mix both `:if` and `:unless` in the same callback declaration: