proc.txt: update kernel filesystem/proc.txt documentation

An update for the "Process-Specific Subdirectories" section to reflect the
changes till kernel 2.6.30.
Signed-off-by: Stefani Seibold <stefani@seibold.net>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>