gen_doc: insert 'generated file' notice

Summary:
Insert a notice that the package group doc files are generated in case
people are tempted to edit them.

Test Plan: generated new docs and previewed on GitHub

Reviewers: oparanal

Reviewed By: oparanal

CC: phab

Differential Revision: https://sibld.phab.dev.bloomberg.com/D145
arcyon query: support 'order' parameter

Summary:
Add support for the 'order' parameter of differential.query so we can
get better 'review per period' statistics.

Test Plan:
$ arcyon query
$ arcyon query --order modified
$ arcyon query --order created

ensure modified is default and created returns sorted ids

Reviewers: oparanal

Reviewed By: oparanal

CC: phab

Differential Revision: https://sibld.phab.dev.bloomberg.com/D143
Fix Arcyd handling of reserved branch

Specifically ignore a special branch 'dev/phab/reserve' when looking for
working branches.

We're using this branch to prevent e.g. someone pushing a 'dev' branch
and blocking creation of all subsequent 'dev/*' branches.

Test Plan:
added 'test_withReservedBranch'
$ ./precommit.sh