prefs-manager: silent some build warnings

- are this really gtk-doc code blocks?

see docs:
https://wiki.gnome.org/Projects/GTK%2B/DocumentationSyntax

Author: raveit65

pluma/pluma-prefs-manager-app.h

@@ -35,7 +35,7 @@
 #include <glib.h>
 #include <pluma/pluma-prefs-manager.h>
 
-/** LIFE CYCLE MANAGEMENT FUNCTIONS **/
+/* LIFE CYCLE MANAGEMENT FUNCTIONS */
 
 gboolean	 pluma_prefs_manager_app_init			(void);
 

pluma/pluma-prefs-manager.h

@@ -129,15 +129,15 @@ typedef enum {
 	PLUMA_TOOLBAR_ICONS_BOTH_HORIZ
 } PlumaToolbarSetting;
 
-/** LIFE CYCLE MANAGEMENT FUNCTIONS **/
+/* LIFE CYCLE MANAGEMENT FUNCTIONS */
 
 gboolean		 pluma_prefs_manager_init (void);
 
 /* This function must be called before exiting pluma */
 void			 pluma_prefs_manager_shutdown (void);
 
 
-/** PREFS MANAGEMENT FUNCTIONS **/
+/* PREFS MANAGEMENT FUNCTIONS */
 
 /* Use default font */
 gboolean 		 pluma_prefs_manager_get_use_default_font 	(void);