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/class-wp-rewrite.php

@@ -1743,7 +1743,7 @@ class WP_Rewrite {
 	 * }
 	 */
 	public function add_permastruct( $name, $struct, $args = array() ) {
-		// Backwards compatibility for the old parameters: $with_front and $ep_mask.
+		// Back-compat for the old parameters: $with_front and $ep_mask.
 		if ( ! is_array( $args ) )
 			$args = array( 'with_front' => $args );
 		if ( func_num_args() == 4 )