document plain auth method

Reference: /n/atom/patch/applied2013/passwd1plain
Date: Tue Jun 18 22:51:56 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/man/1/passwd	Tue Jun 18 22:51:50 2013
+++ /sys/man/1/passwd	Tue Jun 18 22:51:50 2013
@@ -19,6 +19,16 @@
 .B -s
 .I server
 ]
+.B auth/plain
+[
+.B -dn6
+] [
+.B -u
+.I user
+] [
+.B -s
+.I server
+]
 .SH DESCRIPTION
 .I Passwd
 changes the invoker's Plan 9 password and/or APOP secret.
@@ -44,9 +54,11 @@
 It is a substitute for a SecureNet box.
 .PP
 .I Auth/cram
-generates a cram response using
+and
+.I auth/plan
+generates a “cram” or “plain” response using
 .IR factotum (4)
-and a key specified as follows
+and a key specified as follows (example for cram)
 .IP
 .EX
 proto=cram role=client server=%q