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.

Built from https://develop.svn.wordpress.org/trunk@44334


git-svn-id: http://core.svn.wordpress.org/trunk@44164 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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.

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.1-alpha-44333';
+$wp_version = '5.1-alpha-44334';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.