doc/DETAILS contains some very useful information. Unfortunately, pieces of it
are out-of-date (e.g. referring to 2440bis instead of 4880) and it sometimes
conflates the distinct concepts of Ownertrust and User ID Validity by referring
to them both as "trust". Since these concepts are confusing already, it is
probably worth referring to them separately, with different words.

The attached patch updates DETAILS and separates out the concepts more clearly.
I've also fixed a handful of typos and misspellings.