Merge pull request #14059 from tbpgr/guides_active_support_core_ext

[Guides] fix guide active_support_core_extensions. add Note to String#indent [ci skip]

guides/source/active_support_core_extensions.md

@@ -1403,6 +1403,8 @@ The third argument, `indent_empty_lines`, is a flag that says whether empty line
 
 The `indent!` method performs indentation in-place.
 
+NOTE: Defined in `active_support/core_ext/string/indent.rb`.
+
 ### Access
 
 #### `at(position)`