vsnprintf: remove duplicate comment of vsnprintf

Remove the duplicate comment of bstr_printf that is the same as the
vsnprintf.

Add the 's' option to the comment for the pointer function. This is
more of an internal function so the little duplication of the comment
here is OK.

Reported-by: Zhaolei <zhaolei@cn.fujitsu.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>