Cleanup haddock documentation a bit

While grepping for htools imports in the non-htools subdirectory, I
saw that our haddock prologue and title are very very old and refer to
the old htools-only state. Let's cleanup a bit…

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>