Update release playbook, clarify the release announcement section (#701)
Update release playbook, clarify the release announcement section (#701)

- Cosmetic change in `Create a Candidate` (unordered list because we
have to pick one of the 3 options)
- For the release announcement, simplify the review by assigning items to TLs.
- Move editing guidelines to the Google Document, because other people
should follow them.
bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration