Clarify Which Commit Types Cannot Include Breaking Changes

[javier-godoy]

According to the FlowingCode/action-conventional-commits implementation of our Commit Message Guidelines, commit types such as deprecate:, test:, ci:, style: and docs: are not allowed to include breaking changes.

However, this restriction is not explicitly addressed in version 1.0.0-rc.6 of the guidelines.

See also conventional-commits/conventionalcommits.org#536