docs: Fix reference to "bold" part of code example.

Fixes PR14380.

The prose was referring to a "bold" part of the code example, where the
boldness was lost in the transition from HTML. Unlike HTML, where one
can easily have a <b> inside a <pre>, reStructuredText is generally
unable to represent such nested markup.

Hack around it with the :emphasise-lines: option to the  code-block
directive to single out the regions instead. Thankfully the regions are
close-enough to being full lines for this to work.

llvm-svn: 168329