MachO: Add comments describing section flags/attrs

When pruning superfluous MachO structure definitions, I chose to keep
the most generically useful which is why Support's definitions won over
the ones in MC.

However, the MC copy had some useful comments describing some of the
field values.

Bring these back to the copy in Support.  While doing this, fill in some
of the underdocumented definitions as well.

llvm-svn: 209066