fish: add small documentation bit about format of URIs (RHBZ#1450325)

Mention which format URIs follow, with a small example on escaping
special characters.

fish/guestfish.pod

@@ -1225,6 +1225,16 @@ The equivalent API command would be:
 
  ><fs> add /disk protocol:ssh server:tcp:example.com [username:user]
 
+Note that the URIs follow the syntax of
+L<RFC 3986|https://tools.ietf.org/html/rfc3986>: in particular, there
+are restrictions on the allowed characters for the various components
+of the URI.  Characters such as C<:>, C<@>, and C</> B<must> be
+percent-encoded:
+
+ $ guestfish -a ssh://user:pass%40word@example.com/disk.img
+
+In this case, the password is C<pass@word>.
+
 =head1 PROGRESS BARS
 
 Some (not all) long-running commands send progress notification