Remove completed tasks from todo list, add info about git/gerrit preference

Remove completed tasks from todo list, add info about git/gerrit preference

Removed completed tasks (fstrace subcommand help in-binary and issue with
-noexecute vs. -dryrun in vos delentry) from todo list.  Added info about
git/gerrit preference for documentation help, but patches still allowed
to the openafs-doc list.

Change-Id: Ie49ee9a3072372163edf51f7abb246abef894f92
Reviewed-on: http://gerrit.openafs.org/4803
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>