Merge pull request #12215 from koic/update_the_doc_of_style_return_nil

[Docs] Update the doc for `Style/ReturnNil`

lib/rubocop/cop/style/return_nil.rb

@@ -3,9 +3,13 @@
 module RuboCop
   module Cop
     module Style
-      # Enforces consistency between 'return nil' and 'return'.
+      # Enforces consistency between `return nil` and `return`.
       #
-      # Supported styles are: return, return_nil.
+      # This cop is disabled by default. Because there seems to be a perceived semantic difference
+      # between `return` and `return nil`. The former can be seen as just halting evaluation,
+      # while the latter might be used when the return value is of specific concern.
+      #
+      # Supported styles are `return` and `return_nil`.
       #
       # @example EnforcedStyle: return (default)
       #   # bad