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/formatting.php

@@ -934,7 +934,7 @@ function _wp_specialchars( $string, $quote_style = ENT_NOQUOTES, $charset = fals
 
 	$string = @htmlspecialchars( $string, $quote_style, $charset, $double_encode );
 
-	// Backwards compatibility
+	// Back-compat.
 	if ( 'single' === $_quote_style )
 		$string = str_replace( "'", ''', $string );