A guide to the request log lines format. (#8436)

This doc is short but a useful guide to what the request log lines mean.

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
Co-authored-by: Erik Johnston <erik@matrix.org>
Co-authored-by: Daniele Sluijters <daenney@users.noreply.github.com>