suggestion: add a page documenting MSYS options

[jeremyd2019]

Based on https://cygwin.com/cygwin-ug-net/using-cygwinenv.html but updated for changes/additions local to MSYS.
On one of my patches that messed with that updated the source for that doc, but then upstream wanted no option, and when the option was later re-applied in the msys2 fork the docs didn't come with it. The doc isn't necessarily built and put anywhere users would find it anyway. I think a page here would be a better place for it. I might get around to doing this myself, but at least want to record the idea.