Auto merge of #29309 - rjbs:doc-comment-sections, r=alexcrichton

As displayed before this commit, I found the book confusing in its
explanation of `#`-led comments in `rust` blocks.  Possibly the
biggest confusion was because the many-dashes construct does not
become an HR element in the Markdown translator used, so things were
not being properly set off.

This change should more clearly show the as-rendered content as
rendered, and the as-coded content as code.

diff --cc src/doc/trpl/documentation.md
Simple merge