Docs: Standardize on 'backward compatibility/compatible' nomenclature in core inline docs.

Also use 'back-compat' in some inline comments where backward compatibility is the subject and shorthand feels more natural.

Note: 'backwards compatibility/compatibile' can also be considered correct, though it's primary seen in regular use in British English.

Props ocean90.
Fixes #36835.

Built from https://develop.svn.wordpress.org/trunk@37431


git-svn-id: http://core.svn.wordpress.org/trunk@37397 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/theme-compat/comments.php

@@ -4,7 +4,7 @@
  * @subpackage Theme_Compat
  * @deprecated 3.0
  *
- * This file is here for Backwards compatibility with old themes and will be removed in a future version
+ * This file is here for backward compatibility with old themes and will be removed in a future version
  *
  */
 _deprecated_file(

wp-includes/theme-compat/footer.php

@@ -4,8 +4,7 @@
  * @subpackage Theme_Compat
  * @deprecated 3.0
  *
- * This file is here for Backwards compatibility with old themes and will be removed in a future version
- *
+ * This file is here for backward compatibility with old themes and will be removed in a future version
  */
 _deprecated_file(
 	/* translators: %s: template name */

wp-includes/theme-compat/header.php

@@ -4,8 +4,7 @@
  * @subpackage Theme_Compat
  * @deprecated 3.0
  *
- * This file is here for Backwards compatibility with old themes and will be removed in a future version
- *
+ * This file is here for backward compatibility with old themes and will be removed in a future version.
  */
 _deprecated_file(
 	/* translators: %s: template name */

wp-includes/theme-compat/sidebar.php

@@ -4,8 +4,7 @@
  * @subpackage Theme_Compat
  * @deprecated 3.0
  *
- * This file is here for Backwards compatibility with old themes and will be removed in a future version
- *
+ * This file is here for backward compatibility with old themes and will be removed in a future version.
  */
 _deprecated_file(
 	/* translators: %s: template name */