I've created a ticket for this problem now:
https://issues.apache.org/jira/browse/SOLR-13757
Maybe it's just a matter of documentation.
I'm using a workaround now.
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Okay, after a bit of debugging and reading JIRA issues (SOLR-12074,
SOLR-12632), I think this does not work because there's no terms index for
point fields.
So it probably cannot work, and one would need to have a separate string
field with the same values (copyField) to use the termfreq function
