src/server: usage/help/doc updates

Make "usage" messages use the same EBNF as the User's Guide;
no angle brackets.  Improve and correct various helptexts.

Specifically for the port commands, clarify that the number
is optional, and omitting it causes the current number to be
displayed.

Don't use "&function"; a function's name is its address.
Remove a couple instances of pointless whitespace; shrink a
few overlong lines.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

src/server/telnet_server.c

@@ -619,17 +619,18 @@ COMMAND_HANDLER(handle_exit_command)
 static const struct command_registration telnet_command_handlers[] = {
 	{
 		.name = "exit",
-		.handler = &handle_exit_command,
+		.handler = handle_exit_command,
 		.mode = COMMAND_EXEC,
 		.help = "exit telnet session",
 	},
 	{
 		.name = "telnet_port",
-		.handler = &handle_telnet_port_command,
+		.handler = handle_telnet_port_command,
 		.mode = COMMAND_ANY,
-		.help = "port on which to listen "
-			"for incoming telnet connections",
-		.usage = "<port>",
+		.help = "Specify port on which to listen "
+			"for incoming telnet connections.  "
+			"No arguments reports telnet port; zero disables.",
+		.usage = "[port_num]",
 	},
 	COMMAND_REGISTRATION_DONE
 };