Missing documentation for commitType configuration

[brandonlenz]

Describe the bug
A recent all-contributors PR automatically added an autoconfiguration for commitType, but at the time of writing it is not noted as a possible configuration in the official docs

To Reproduce
Steps to reproduce the behavior:

1. Use the all contributors bot to open a PR on a repo (might be relevant that the configuration specifies angular as the commitConvention)
2. Note that commitType is updated to a default of docs in this commit by AC bot

Expected behavior
Documentation should list commitType description and accepted options/values (or link to them if pulling from conventional commits spec)

Anything else?

Relevant PR adding the change to the bot: all-contributors/app#462

It is specifically noted in the PR body that due to lack of documentation on commitConvention, docs for commitType were skipped as well. There are, however, docs for commitConvention here, so there should be docs for commitType as well

[brandonlenz]

cc: @tobiasbueschel

[bressler1995]

I also have to second this. I am going to be merging from a fork to another repository, and this commitType variable was added automatically to the configuration. It would be good to have some documentation on this since it is not exactly known what behavior this may result in. Hopefully it's something insignificant.

[danieltott]

Got a PR up for this here: #757