Documentation: git-send-mail can take rev-list arg to drive format-patch

The git-send-email docs do not mention except in the usage lines
the combined patch formatting/sending ability of git-send-email.
This patch expands on the possible arguments to git-send-email
and explains the meaning of the rev-list argument.

Signed-off-by: Paolo Bonzini <bonzini@gnu.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-send-email.txt

@@ -14,6 +14,10 @@ SYNOPSIS
 DESCRIPTION
 -----------
 Takes the patches given on the command line and emails them out.
+Patches can be specified as files, directories (which will send all
+files in the directory), or directly as a revision list.  In the
+last case, any format accepted by linkgit:git-format-patch[1] can
+be passed to git send-email.
 
 The header of the email is configurable by command line options.  If not
 specified on the command line, the user will be prompted with a ReadLine