Allow # to appear in rustdoc code output.
"##" at the start of a trimmed rustdoc line is now cut to "#" and then
shown. If the user wanted to show "##", they can type "###".
I'm somewhat concerned about the potential implications for users, since this does make a potentially backwards-incompatible change. Previously, `##` had no special handling, and now we do change it. However, I'm not really sure what we can do here to improve this, and I can't think of any cases where `##` would likely be correct in a code block, though of course I could be wrong.
Fixes#41783.
ced823e267