Docs: Improve the DocBlock for validate_plugin().

Props theMikeD.
Fixes #36333.

git-svn-id: https://develop.svn.wordpress.org/trunk@39889 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/plugin.php

@@ -940,11 +940,11 @@ function validate_active_plugins() {
 /**
  * Validate the plugin path.
  *
- * Checks that the file exists and is a valid file. See validate_file().
+ * Checks that the main plugin file exists and is a valid plugin. See validate_file().
  *
  * @since 2.5.0
  *
- * @param string $plugin Plugin Path.
+ * @param string $plugin Path to the main plugin file from plugins directory.
  * @return WP_Error|int 0 on success, WP_Error on failure.
  */
 function validate_plugin($plugin) {