Docs: Add backtick escaping around a variable name in the DocBlock for `update_post_meta()`.

Also adds a note to the description explaining that it can be used in place of `add_post_meta()`.

See #42505.


git-svn-id: https://develop.svn.wordpress.org/trunk@44334 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/post.php

@@ -2027,11 +2027,13 @@ function get_post_meta( $post_id, $key = '', $single = false ) {
 /**
  * Updates a post meta field based on the given post ID.
  *
- * Use the $prev_value parameter to differentiate between meta fields with the
+ * Use the `$prev_value` parameter to differentiate between meta fields with the
  * same key and post ID.
  *
  * If the meta field for the post does not exist, it will be added and its ID returned.
  *
+ * Can be used in place of add_post_meta().
+ *
  * @since 1.5.0
  *
  * @param int    $post_id    Post ID.