Minor documentation improvements

Mostly correcting it's to its and changing some , to .

gdk/gdkwindow.c

@@ -78,7 +78,7 @@
  * composited and therefore is no longer automatically drawn to the screen.
  *
  * When the contents of the event box change, an expose event is generated on
- * it's parent window (which, in this case, belongs to the toplevel #GtkWindow).
+ * its parent window (which, in this case, belongs to the toplevel #GtkWindow).
  * The expose handler for this widget is responsible for merging the changes
  * back on the screen in the way that it wishes.
  *

gdk/quartz/gdkeventloop-quartz.c

@@ -874,7 +874,7 @@ run_loop_before_waiting (void)
     }
   else if (timeout > 0)
     {
-      /* We need to get the run loop to break out of it's wait when our timeout
+      /* We need to get the run loop to break out of its wait when our timeout
        * expires. We do this by adding a dummy timer that we'll remove immediately
        * after the wait wakes up.
        */

gdk/win32/gdkkeys-win32.c

@@ -417,7 +417,7 @@ update_keymap (void)
 		    {
 		      guint keysym = gdk_unicode_to_keyval (wcs[0]);
 
-		      /* It is a dead key, and it's has been stored in
+		      /* It is a dead key, and it has been stored in
 		       * the keyboard layout's state by
 		       * ToAsciiEx()/ToUnicodeEx(). Yes, this is an
 		       * incredibly silly API! Make the keyboard