committer's guide: document MFC to: and MFC with:

There are a couple of different forms of these in use. Add "MFC to:" and
"MFC with:" tags consistent with the existing "MFC after:", and provide
a description so that consistent values can be used.

There are ongoing discussions about "X-MFC-*" tags which still need to be
resolved and documented here, or general guidance on X-MFC-*.

Reviewed by: bcr, ngie
Differential Revision: https://reviews.freebsd.org/D10585