Add a few observations to the merge-tracking requirements.
Add a few observations to the merge-tracking requirements.

* www/merge-tracking/requirements.html
  (repeated-merge): Note that svk doesn't really support
   cherry-picking in sense compatible with repeated merge.
  (cherry-picking): Note that cherry-picking in multiple directions is
   difficult.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion