pgsql: Add description for backend termination: < cleaned up

Log Message:
-----------
Add description for backend termination:

< cleaned up properly. A new signal is needed for safe termination.

cleaned up properly. A new signal is needed for safe termination
because backends must first do a query cancel, then exit once they
have run the query cancel cleanup routine.

Modified Files:
--------------
pgsql/doc:
TODO (r1.1551 -> r1.1552)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/TODO.diff?r1=1.1551&amp;r2=1.1552)
pgsql/doc/src/FAQ:
TODO.html (r1.58 -> r1.59)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/FAQ/TODO.html.diff?r1=1.58&amp;r2=1.59)