minor doc tweak

Per a user-submitted comment on the documentation, it would be nice if
the breadcrumbs at the top of the documentation pages provided a link
back to the top-level page for the current set of manuals being viewed.
In other words, to pick a page at random:

http://www.postgresql.org/docs/8.0/interactive/functions.html

has "Home -> Documentation -> Manuals -> PostgreSQL 8.0" at the top, but
the "PostgreSQL 8.0" is not linked. For users who arrive in the midst of
the documentation via Google, it would be nice to make it easier for
them to get back to the ToC for the 8.0 docs without needing to go to
"Manuals" first.

-Neil