pgsql: Re-add TODO and clarify it is for the kernel cache: < * Allow

Log Message:
-----------
Re-add TODO and clarify it is for the kernel cache:

< * Allow free-behind capability for large sequential scans, perhaps using
< posix_fadvise()

* Allow free-behind capability for large sequential scans to avoid
kernel cache spoiling

Modified Files:
--------------
pgsql/doc:
TODO (r1.2198 -> r1.2199)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/TODO.diff?r1=1.2198&amp;r2=1.2199)
pgsql/doc/src/FAQ:
TODO.html (r1.699 -> r1.700)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/FAQ/TODO.html.diff?r1=1.699&amp;r2=1.700)