send-email: further document missing sendmail aliases functionality

Sendmail aliases[1] supports expansion to a file ("/path/name") or
pipe ("|command"), as well as file inclusion (":include: /path/name"),
however, our implementation does not support such functionality.

[1]: https://www.freebsd.org/cgi/man.cgi?query=aliases&sektion=5

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-send-email.txt

@@ -396,6 +396,9 @@ sendmail;;
 	contain a `"` symbol are ignored.
 *	Line continuations are not supported: lines that start with
 	whitespace characters, or end with a `\` symbol are ignored.
+*	Redirection to a file (`/path/name`) or pipe (`|command`) is not
+	supported.
+*	File inclusion (`:include: /path/name`) is not supported.
 *	Warnings are printed on the standard error output for any
 	explicitly unsupported constructs, and any other lines that are not
 	recognized by the parser.