multiline: single backtick a little annoying

[Reinier Zwitserloot]

This is very much a nitpick, but allowing a single backtick does mean they
aren't really raw; backticks can show up rather easily. I would then have
to go find the delimiters and add a second backtick.

Making double-backtick mandatory solves this problem.

One strength of the triple-quote solution is that triple quote is far more
rare than a backtick. It would also solve this nitpick.

 --Reinier Zwitserloot