Update committers-doc.dot, noving doc committer alumni into the alumni

Update committers-doc.dot, noving doc committer alumni into the alumni
section and recording the date these commit bits were returned.  Also, sort
the remaining entries, and add a node for mheinen, who is already referenced
within the mentor relationships.

Other doc committers are encouraged to add themselves, and any mentor/mentee
relationships.  The current rendered output is available at
http://people.freebsd.org/~gavin/comm-doc.png