Manual pages fixups

[kdave]

https://lore.kernel.org/linux-btrfs/ZeRuEH029j3enMr0@meinfjell.helgefjelltest.de/

There are many reported issues, noted in the formatting of the final manual pages. Most of them seem to be related to rendering of other manual pages links, which are now using a common formatter so this should be easily fixable.

[kdave]

Updating the manref role does not work, it's not called when rendering manual pages. It should be possible to either add a visitor or override the manual page builder.