get_maintainer: add missing documentation for --git-blame-signatures

I really haven't used this option much myself, so feel free to improve on
the documentation for it.  I just noticed it while inspecting this script
for undocumented features.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>