Our Join method is different to pandas Join

We overload the pandas `join` method, using it for our spatial join. Instead, any traditional joining is done using the `merge` function. Users get confused with differences between our behavior and pandas, especially when this will give an incorrect result if users expect to get a full table join instead of a spatial one. We could consider renaming our methods, or making it more clear to users that this is a spatial join.

Start by reviewing the current spatial join and merge APIs, then compare their behavior with pandas' traditional join semantics. The issue leaves the solution open between renaming methods and clarifying the existing API; done should mean users are not likely to mistake the spatial join for a full table join.

Written by the indexing model from the issue text.