Document the use of spatial functions in primaryjoin conditions

[elemoine]

Fixes #220.

[elemoine]

I'll merge this this afternoon or tomorrow if I there's no feedback.

[troyswanson]

I wonder if it would be helpful to provide an example the other way as well, to show the relationship from the treasure's perspective.

In other words, "what lake does this treasure live in?" vs "what treasure lives in this lake?"

Also, it might be helpful to discuss (very basic) lazy/eager loading strategies. With lazy loading, Iterating on the lake response and accessing the treasure attribute would get very expensive, since it would emit a new SQL query for every lake. Eager loading would eliminate that expense.

Related docs: https://docs.sqlalchemy.org/en/13/orm/loading_relationships.html#joined-eager-loading

[elemoine]

I can add a backref to the relationship and a sentence to explain what it does.

I can also mention that the relationship is configured with the default lazy="select" strategy, with a link to the appropriate SQLA doc section.

And I think I'll stop there, to avoid rewriting the SQLA docs :)

Thanks for your suggestions!

[elemoine]

I added more information. Will merge when the CI process is finished.