mac80211: rewrite Kconfig text for mesh

Lubomir Rintel recently pointed out a dead link for o11s.org, and
repointed it to a still live, but also stale website.  As far as I
know, no one is updating the content at open80211s.org.

Since this Kconfig text was originally written, though, the 802.11s
mesh drafts were approved and ultimately rolled into 802.11 proper.
Meanwhile, the implementation has converged on the final standard,
so we can lose all of the text here and provide something that's a
little more helpful and accurate.

Signed-off-by: Bob Copeland <bobcopeland@fb.com>
Reviewed-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Steve deRosier <derosier@cal-sierra.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>