[docs] Fixing Sphinx warnings to unclog the buildbot

Lots of blocks had "llvm" or "nasm" syntax types but either weren't following
the syntax, or the syntax has changed (and sphinx hasn't keep up) or the type
doesn't even exist (nasm?).

Other documents had :options: what were invalid. I only removed those that had
warnings, and left the ones that didn't, in order to follow the principle of
least surprise.

This is like this for ages, but the buildbot is now failing on errors. It may
take a while to upgrade the buildbot's sphinx, if that's even possible, but
that shouldn't stop us from getting docs updates (which seem down for quite
a while).

Also, we're not losing any syntax highlight, since when it doesn't parse, it
doesn't colour. Ie. those blocks are not being highlighted anyway.

I'm trying to get all docs in one go, so that it's easy to revert later if we
do fix, or at least easy to know what's to fix.

llvm-svn: 276109