Docs: Consistently include an empty line between @since tag and @see, @link, or @global, per the documentation standards.

See #49572.

git-svn-id: https://develop.svn.wordpress.org/trunk@48067 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/plugin.php

@@ -542,6 +542,7 @@ function is_plugin_active( $plugin ) {
  * Conditional Tags} article in the Theme Developer Handbook.
  *
  * @since 3.1.0
+ *
  * @see is_plugin_active()
  *
  * @param string $plugin Path to the plugin file relative to the plugins directory.