documentation output directory

Hi,

In looking at snapshot generation it becomes clear that the output of
the documentation is misplaced.  I say we should move the output dirs to
be:

doc/html
doc/man/man1
doc/man/man3
doc/man/man7
doc/src/...

Objections?  Peter, I think you're working on other doc-related things
(build dir rather than source dir?), so I'm not going to touch this
until you tell me it's OK to do so.

--
Alvaro Herrera                                http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support