Administration: Replace contracted verb forms for better consistency.

This changeset replaces contracted verb forms like `doesn't`, `can't`, or `isn't` with non-contracted forms like `does not`, `cannot`, or `is not`, for better consistency across the WordPress administration. It also updates some corresponding unit tests strings.

Props Presskopp, socalchristina, aandrewdixon, francina, SergeyBiryukov, JeffPaul, audrasjb, hellofromTonya.
Fixes #38913.
See #39176.


git-svn-id: https://develop.svn.wordpress.org/trunk@52978 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/post.php

@@ -1713,7 +1713,7 @@ function register_post_type( $post_type, $args = array() ) {
 /**
  * Unregisters a post type.
  *
- * Can not be used to unregister built-in post types.
+ * Cannot be used to unregister built-in post types.
  *
  * @since 4.5.0
  *