db/DatabasePlugin: add #EventLoop parameter documentation

src/db/DatabasePlugin.hxx

@@ -46,8 +46,12 @@ struct DatabasePlugin {
 	 * Allocates and configures a database.
 	 *
 	 * Throws #std::runtime_error on error.
+	 *
+	 * @param main_event_loop the #EventLoop running in the same
+	 * thread which invokes #Database methods
 	 */
-	Database *(*create)(EventLoop &loop, DatabaseListener &listener,
+	Database *(*create)(EventLoop &main_event_loop,
+			    DatabaseListener &listener,
 			    const ConfigBlock &block);
 
 	constexpr bool RequireStorage() const {