pgsql: Make log_temp_files based on kB, and revert docs & comments to

Log Message:
-----------
Make log_temp_files based on kB, and revert docs & comments to match.

Per extensive discussion on pgsql-hackers. We are deliberately not
back-patching this even though the behavior of 8.3 and 8.4 is
unquestionably broken, for fear of breaking existing users of this
parameter. This incompatibility should be release-noted.

Modified Files:
--------------
pgsql/doc/src/sgml:
config.sgml (r1.292 -> r1.293)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.292&r2=1.293)
pgsql/src/backend/storage/file:
fd.c (r1.156 -> r1.157)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/file/fd.c?r1=1.156&r2=1.157)
pgsql/src/backend/utils/misc:
guc.c (r1.560 -> r1.561)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.560&r2=1.561)