changes to documentation build

Per thread starting here

http://archives.postgresql.org/pgsql-bugs/2010-03/msg00049.php

and ending here

http://archives.postgresql.org/pgsql-bugs/2010-03/msg00110.php

and earlier discussions of similar nature, here is a patch that
separates the documentation build and installation into separate
top-level targets 'make docs' and 'make install-docs', also dependencies
of 'make world' and 'make install-world'.

The significant change is that the documentation is no longer installed
by default (but the availability of the 'world' target might alleviate
that).

Please check it out.