Document quirks with xdg-mime for query file and query default

This documents the wired behavior for xdg-mime query default in relation to xdg-open (#147) as that won't be possible to fix in the near future and the quirk that xdg-mime does not like filenames that start with a minus sign and how to work around that.

The upgrade to DocBook 4.3 is necessary for the <code> tags to work.

Please apply together with !81 (merged), the MRs shouldn't conflict.