#8 Incorrect cross reference warning

[Joachim Schrod]

In wegweiser index, we get warnings

ERROR: Cross-reference-target ("Trennung") does not exist!

This entry does exist, but only as top-level anchor for
sub-entries,
without an own location.

[Joachim Schrod]

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user_id=70318

Missing cross reference targets should not be an error
anyhow, but should issue a warning.
Btw, the index is created correctly, that's why this ticket
has no high priority.