www/falkon: add hint and resolution to color problems in video playback

- when hardware accelartion is not available users can experience switched colors
- to work around the issue it is possible to pass --disable-gpu to the command line

PR:		240095
PR:		237277
Submitted by:	Lorenzo Salvadore
Reported by:	Nuno Teixeira

www/falkon/pkg-message

@@ -5,6 +5,11 @@ On FreeBSD < 11.3 Web Content process may frequently crash due to bug 181741.
 If you cannot upgrade try applying the following workaround:
 
 # sysctl net.local.stream.recvspace=16384
+
+If you get wrong colors when watching videos, this is probably because your
+system does not support hardware acceleration (see bug 237277): you can
+disable it by starting the browser from command line with the --disable-gpu
+option.
 EOM
 }
 ]