Gendoc now extracts common prefixes of overlong headers

The overlong headers arise when there are lots of words with identical
description, such as will necessarily be the case for all the words of
the sort ?AT_*, !DW_AT_* etc.  The gendoc now acutally presents these
words unders such abbreviated headings.
Show all the hidden documentation bulitin-dw

This was hidden because it messed up the documentation layout.  As of
last commit, gendoc can handle this and these words are now rendered
fairly reasonably.