Fix spawn documentation so it appears in the API docs

doc/plugins.dox

@@ -55,6 +55,7 @@ We will try to document as many functions and structs as possible.
 - @link msgwindow.h @endlink
 - @link project.h @endlink
 - @link sciwrappers.h Scintilla Wrapper Functions @endlink
+- @link spawn.h Spawning programs @endlink
 - @link stash.h Stash Pref/Setting Functions @endlink
 - @link utils.h General Utility Functions @endlink
 - @link ui_utils.h Widget Utility Functions @endlink

src/spawn.c

@@ -39,6 +39,10 @@
  * This module does not depend on Geany when compiled for testing (-DSPAWN_TEST).
  */
 
+/** @file spawn.h
+ * Portable and convenient process spawning and communication.
+ */
+
 #ifdef HAVE_CONFIG_H
 # include "config.h"
 #endif