[m-rev.] for post-commit review: explain grades more deeply

[Zoltan Somogyi]

This started out as clarification of .spf, but snowballed from there.

For post-commit review (in the form of changes to the file) by anyone.
I am particularly seeking feedback about the description of the grade
modifiers dealing with threads.

Two related issues. First, having the description of the whole grade system
be inside the item for the --grade option in a table of options seems wrong.
I would prefer if we had a whole subsubsection for it, where 9.8.0 would be
if 0 was a possible subsubsection number. Opinions?

Second, the user guide still mentions the --trail-segments option,
and the compiler still has such an option. I propose that we delete
all mention of --trail-segments option from both the user guide
and the compiler, with one possible exception, that being that
"trail-segments" would become an alternate name for the use_trail
option. That would enable any obsolete reference in Mmakefiles
to still work. Julien, is this accommodation of old Mmakefiles
still needed? Has there been enough time since the old-style .tr grade
has been retired to remove all references to trail segments?

Zoltan.
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