[m-rev.] for post-commit review: add "opt level N sets --xyz" to the user guide

[Zoltan Somogyi]

While Julien objected to adding this info to the help text,
I don't believe there were any objections to adding it to the
user guide.

Note that this diff requires one of the compiler bug fixes I posted
yesterday. The rotd containing that fix just became available.

Zoltan.
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