docs: Attempt to fix PR14053.

Hypothesis 1: use of `.. code::` directive instead of `.. code-block::`
is causing Sphinx to discard the block. On my machine, `.. code::`
renders fine. However, I don't think that `..  code::` is actually a
legit Sphinx directive. I believe that on my machine Sphinx is falling
back to just displaying it monospace with no syntax, whereas llvm.org's
Sphinx is just discarding it.

This is truly "remote debugging" since I can't reproduce this on my
machine. It would be helpful to be able to see the llvm.org Sphinx
build logs; if that's possible please let me know.

llvm-svn: 165632