lib/vsprintf.c: document more vsnprintf extensions

lib/vsprintf.c: document more vsnprintf extensions

These were added in

cc9ea8e (lib/vsprintf.c: add %pU to print UUID/GUIDs)
f44257f (vsprintf: use %pR, %pr instead of %pRt, %pRf)
6f69ece (lib/vsprintf.c: Add "%pI6c" - print pointer as compressed ipv6 address)
cfdd898 (printk: add %I4, %I6, %i4, %i6 format specifiers)
b455ef3 (printk: add %pM format specifier for MAC addresses)

but only added comments to pointer() not vsnprintf() that is refered to by
printk's comments.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Harvey Harrison <harvey.harrison@gmail.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Joe Perches <joe@perches.com>
Cc: Jens Rosenboom <jens@mcbone.net>
Cc: David S. Miller <davem@davemloft.net>
Cc: Bjorn Helgaas <bjorn.helgaas@hp.com>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>