pgsql: Update mmap details: < portability issues.

Log Message:
-----------
Update mmap details:

< portability issues. Anonymous mmap is required to prevent I/O
< overhead.
> portability issues. Anonymous mmap (or mmap to /dev/zero) is required
> to prevent I/O overhead.
>
> * Consider mmap()'ing files into a backend?
>
> Doing I/O to large tables would consume a lot of address space or
> require frequent mapping/unmapping. Extending the file also causes
> mapping problems that might require mapping only individual pages,
> leading to thousands of mappings. Another problem is that there is no
> way to _prevent_ I/O to disk from the dirty shared buffers so changes
> could hit disk before WAL is written.

Modified Files:
--------------
pgsql/doc:
TODO (r1.1390 -> r1.1391)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/TODO.diff?r1=1.1390&r2=1.1391)