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.


git-svn-id: https://develop.svn.wordpress.org/trunk@37431 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/plugin.php

@@ -487,7 +487,7 @@ function is_plugin_active_for_network( $plugin ) {
  * be activated only as a network wide plugin. The plugin would also work
  * when Multisite is not enabled.
  *
- * Checks for "Site Wide Only: true" for backwards compatibility.
+ * Checks for "Site Wide Only: true" for backward compatibility.
  *
  * @since 3.0.0
  *