player: tell user about --force-seeking if demuxer is not seekable

This comes up often, see e.g. #3220. The issue is that if the stream
input is not seekable, the demuxer is marked as not seekable. But if the
stream cache is enabled, the file still _might_ be seekable to a degree.
We recently disabled seeking in this mode because it can cause very
weird issues, mostly because if stream-layer seeking fails, the demuxers
will arbitrarily misbehave. On the other hand, it can work if the seek
is within the cached range, which is why the user can still enable it
with --force-seeking. There is a weird trade-off between allowing this
and not crapping up too easily, so just informing the user about the
possibility seems best.

player/playloop.c

@@ -189,7 +189,8 @@ static int mp_seek(MPContext *mpctx, struct seek_params seek)
         return -1;
 
     if (!mpctx->demuxer->seekable) {
-        MP_ERR(mpctx, "Can't seek in this file.\n");
+        MP_ERR(mpctx, "Cannot seek in this file.\n");
+        MP_ERR(mpctx, "You can forcibly enable it with '--force-seeking=yes'.\n");
         return -1;
     }