FSM rewrite: doc changes

To keep everyone who's interested up-to-date, attached is the latest
patch. The big addition is documentation. I added a section in the
"Physical Database Layout" chapter about the new FSM, and removed
references to the max_fsm_relations/pages options, plus some other small
changes.

I also moved some the one debugging function to peek into an FSM page at
low level to contrib/pageinspect, and updated the documentation of
pg_freespacemap and pageinspect to match the code.

I find it a bit disturbing that a documentation patch actually removes
more lines from the manual than adds, but it's quite understandable
because it's no longer necessary to explain the two GUC options that
used to be quite important :-). Comments welcome.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com