send-email: document the --smtp-debug option
send-email: document the --smtp-debug option

The option was already shown in -h output, so it should be documented
in the man page.

Signed-off-by: Zbigniew Jędrzejewski-Szmek <1a002461b1932d3892b64879e6e8987274fa7074@in.waw.pl>
Suggested-by: David A. Greene <699889fa4d775c82b36574aaa0f8ebb772e0c366@obbligato.org>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git