Improve documentation for `deps` et al

**Is your feature request related to a problem? Please describe.**

The section [Release Please bot does not create a release PR. Why?](https://github.com/googleapis/release-please#release-please-bot-does-not-create-a-release-pr-why) refers to `deps` as a conventional commit `type`. This makes `deps` a quite important prefix when using conventional commits. Though, conventionalcommits.org and very few other google searches refers to `deps` as a commit type. It would be helpful if the release please readme clarifies which prefixes has syntactic meaning, and some recommendation on how to use it.