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&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&r2=1.700)