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-oembed.php

@@ -38,7 +38,7 @@ class WP_oEmbed {
 	public static $early_providers = array();
 
 	/**
-	 * A list of private/protected methods, used for backwards compatibility.
+	 * A list of private/protected methods, used for backward compatibility.
 	 *
 	 * @since 4.2.0
 	 * @access private
@@ -192,7 +192,7 @@ class WP_oEmbed {
 	}
 
 	/**
-	 * Exposes private/protected methods for backwards compatibility.
+	 * Exposes private/protected methods for backward compatibility.
 	 *
 	 * @since 4.0.0
 	 * @access public