Do not handle the "%" char in a special way, in the fixed_server mode

Also fix some doc

doc/biboumi.1.md

@@ -63,15 +63,19 @@ The configuration file uses a simple format of the form
 
   If this option contains the hostname of an IRC server (for example
   irc.example.org), then biboumi will enforce the connexion to that IRC
-  server only.  This means that a JID like "#chan@irc.biboumi.org" must be
-  used instead of "#chan%irc.example.org@irc.biboumi.org".  In that mode,
-  the virtual channel (see *Connect to an IRC server*) is not available and
-  you still need to use the ! separator to send message to an IRC user (for
-  example "foo!@biboumi.example.com" to send a message to foo), although the
-  in-room JID still work as expected ("#channel@biboumi.example.com/Nick").
-  This option can for example be used by an administrator that just wants to
-  let their users join their own IRC server using an XMPP client, but
-  without letting them join any other IRC servers on the internet.
+  server only.  This means that a JID like "#chan@biboumi.example.com" must
+  be used instead of "#chan%irc.example.org@biboumi.example.com".  In that
+  mode, the virtual channel (see *Connect to an IRC server*) is not
+  available and you still need to use the ! separator to send message to an
+  IRC user (for example "foo!@biboumi.example.com" to send a message to
+  foo), although the in-room JID still work as expected
+  ("#channel@biboumi.example.com/Nick").  On the other hand, the '%' lose
+  any meaning.  It can appear in the JID but will not be interpreted as a
+  separator (thus the JID "#channel%hello@biboumi.example.com" points to the
+  channel named "#channel%hello" on the configured IRC server) This option
+  can for example be used by an administrator that just wants to let their
+  users join their own IRC server using an XMPP client, while forbidding
+  access to any other IRC server.
 
 `log_file`