[m-rev.] for post-commit review: user guide improvements

[Zoltan Somogyi]

For post-commit review by Julien. Could you please update
the parts about non-C grades and mmake?

Zoltan.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Log.ug
Type: application/octet-stream
Size: 847 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20200530/5b892aab/attachment-0002.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: DIFF.ug
Type: application/octet-stream
Size: 16034 bytes
Desc: not available
URL: <http://lists.mercurylang.org/archives/reviews/attachments/20200530/5b892aab/attachment-0003.obj>