Docs: Corrections and improvements to types used in various docblocks.

See #51800, #52217


git-svn-id: https://develop.svn.wordpress.org/trunk@49936 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/plugin.php

@@ -1231,7 +1231,8 @@ function is_uninstallable_plugin( $plugin ) {
  * @since 2.7.0
  *
  * @param string $plugin Path to the plugin file relative to the plugins directory.
- * @return true True if a plugin's uninstall.php file has been found and included.
+ * @return true|void True if a plugin's uninstall.php file has been found and included.
+ *                   Void otherwise.
  */
 function uninstall_plugin( $plugin ) {
 	$file = plugin_basename( $plugin );