do ./gendoc.sh missed on earlier commmit

Test Plan: $ ./precommit.sh

Reviewers: jevripio

Reviewed By: jevripio

CC: arcyd

Differential Revision: https://sibld.phab.dev.bloomberg.com/D177
Arcyd: fix revision update comment

Summary:
When updating an existing revision, make sure that the first line of the
comment contains something that wouldn't look out of place in the
"Revision Update History" log.

Test Plan:
$ ./precommit.sh
inspect 'happy path' in
$ ./testbed/arcyd/acceptance_tests.sh

Reviewers: jevripio

Reviewed By: jevripio

CC: arcyd

Maniphest Tasks: T35

Differential Revision: https://sibld.phab.dev.bloomberg.com/D175