Add ORM documentation for as_comparison()

Add ORM documentation for as_comparison()

In #3831 we added the ability for SQL functions to be used in
primaryjoin conditions as the source of comparison, however we
didn't add documentation from the main relationship docs so
nobody could find it unless they read the migration notes.
Since the two use cases that have come up for this are
materialized path with string functions, and geometry functions,
add the example based on the use case requested in
https://github.com/geoalchemy/geoalchemy2/issues/220#issuecomment-486709055

This example hasn't been tested yet and is subject to
revision.

Change-Id: I410d8ffade3f17cf616fc5056f27d7d32092207b