If you omit linktext the topic ID appears as link text, as in the following example.
// Foo does nothing meaningful, it just
// calls <link Bar> and
// that's all.
----------
// Foo does nothing meaningful, it just
// calls <xref target= "Bar"/> and
// that's all.
void Foo();
// Bar is called by Foo and returns a
// checksum for the argument s.
int Bar(char *s);
