Allow better control over the release notes/CHANGELOG

[chingor13]

Sometimes repository maintainers need to tweak the published release notes and changelog entries for either correctness or readability. Currently, the release notes are rewritten every time release-please runs.

[plamut]

I was just about to report this.

In addition, in our concrete case the bot included a release note about a PR that was subsequently reverted. Here's the release PR:
googleapis/python-bigquery#756

The problematic item is the first one in the Features section, i.e. "add always_use_jwt_acces". That PR was subsequently reverted, but the bot did not detect that. Should have been the reversal PR prefixed with revert: instead?

(if this is indeed a bug that warrants a separate issue, let me know)

[chingor13]

Proposal: Add a parsable section to the merged pull request that overrides the commit message for that commit.

Example:

BEGIN_COMMIT_OVERRIDE
fix: rewritten commit message
docs: another commit message
END_COMMIT_OVERRIDE

This does not grant full control over the release notes, but should be good enough for most cases.