pgsql: Update TODO description: * Speed WAL recovery by allowing more

Log Message:
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Update TODO description:

* Speed WAL recovery by allowing more than one page to be prefetched

This should be done utilizing the same infrastructure used for
prefetching in general to avoid introducing complex error-prone code
in WAL replay.

Modified Files:
--------------
pgsql/doc:
TODO (r1.2348 -> r1.2349)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/TODO?r1=1.2348&r2=1.2349)
pgsql/doc/src/FAQ:
TODO.html (r1.856 -> r1.857)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/FAQ/TODO.html?r1=1.856&r2=1.857)