doc: properly handle preformatted blocks

[whitslack]

Lowdown requires a blank line before all preformatted blocks, or it doesn't recognize them. tools/md2man.sh contained some ad-hoc efforts at fixing up some locations where these required blank lines are absent from the output of tools/fromschema.py, but it missed some. Instead of playing Whack-a-Mole, use a blanket sed expression to ensure that a blank line precedes every opening ```.

esc_underscores(…) in tools/fromschema.py did not work correctly on strings containing an odd number of backticks, notably the ``` delimiters surrounding preformatted text blocks. Specifically, it was dropping the last backtick since none of the alternatives in the regex matched it. Add a new alternative that matches a whole preformatted block as a single unit.

output_member(…) in tools/fromschema.py was passing each line of a member's description through esc_underscores(…) individually, but that breaks preformatted text blocks that are naturally multi-line and leads to mistakenly escaping underscores inside such blocks. Rewrite the code to make use of the outputs(…) utility function that joins all the provided lines together before passing the whole text through esc_underscores(…).

Drive-by fix a couple of flubbed preformatted blocks in schemas.

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