Add hint to prevent double calling of main_local_init() into plugin HowTo.

git-svn-id: https://geany.svn.sourceforge.net/svnroot/geany/trunk@5880 ea778897-0a13-0410-b9d1-a72fbfd435f5

doc/plugins.dox

@@ -451,4 +451,7 @@ void plugin_init(GeanyData *data)
 		G_CALLBACK(item_activate_cb), NULL);
 }
  * @endcode
+ *
+ * If you already did use PLUGIN_SET_TRANSLATABLE_INFO() you don't need
+ * to add main_locale_init() as it has been already called.
  **/