man: update redfishpower(8) with plug substitution

Problem: redfishpower is out of date with recent updates
supporting plug substituation in paths.

Document use of %p when setting plugs via setplugs.

man/redfishpower.8.in

@@ -62,7 +62,12 @@ Set default path and optional post data to turn off plug.
 .TP
 .I "setplugs plugnames hostindexes"
 Set plug names for index of hosts input to redfishpower.  Can be
-called multiple times to configure all possible targets.
+called multiple times to configure all possible targets.  In most
+cases the number of plugs should equal the number of indices.
+Multiple plugs can be mapped to a single host index if used with plug
+name substitution.  The plug name can be substituted into the URI path
+by specifying "%p" when configuring the path.  Reminder that "%p"
+often has to be escaped, i.e. "%%p".
 .TP
 .I "setpath <plugnames> <cmd> <path> [postdata]"
 Set path for specific plug power command ("stat", "on", "off") and optional post data.