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)