Add help text for mmacosx-version-min=, mios-version-min=.

I keep forgetting the exact spelling of -macosx-version-min=, and now I can
run `bin/clang --help | grep version -A 2` to remind myself.  While here,
also document -mios-version-min=.  Don't document -mios-simulator-version-min=
as it's just an alias for -mios-version-min= these days.

llvm-svn: 225409