add documentation of MSYS variable settings

This is a proposal of something I want to add somewhere, but haven't figured out exactly how/where. I want to make documentation of the MSYS environment variable options that differ from upstream's https://cygwin.com/cygwin-ug-net/using-cygwinenv.html. I'm responsible for two of them now ☹️ (winjitdebug which I went into detail about using on #116, and now nonativeinnerlinks which doesn't really merit its own page). I'd almost want to patch the docs in msys2-runtime to document new flags, but as far as I know nothing's done with that to put it anywhere a user's likely to find it.

/cc @dscho as Git for Windows either already has something like this or would benefit from it also.

Start by comparing the upstream Cygwin environment-variable documentation with the MSYS-specific behavior described in this issue and discussion #116. Review how msys2-runtime documentation is currently surfaced, then identify a discoverable location for the MSYS options; done means documenting winjitdebug and nonativeinnerlinks and explaining their differences from upstream.