experimental/documentation/gerrit.md: improvements

https://skia.googlesource.com/skia/+/master/experimental/documentation/gerrit.md
NOTRY=true
Change-Id: I90d8e16e9d6146fefe0199bb98d5f8eea31d4e8f
Reviewed-on: https://skia-review.googlesource.com/7750
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
2 files changed