tinc: Mention in docs that the host name may not be used verbatim. (#26157)

* tinc: Mention in docs that the host name may not be used verbatim.

Source:

  5c344f2976/src/net_setup.c (L341)

* tinc: also replaces non-alphanumeric characters.

nixos/modules/services/networking/tinc.nix

@@ -35,7 +35,8 @@ in
               description = ''
                 The name of the node which is used as an identifier when communicating
                 with the remote nodes in the mesh. If null then the hostname of the system
-                is used.
+                is used to derive a name (note that tinc may replace non-alphanumeric characters in
+                hostnames by underscores).
               '';
             };