test: document join scoping for a region absent from the query

[jan-glx]

Pins down current behavior for a table row whose region wasn't part of spatial_element_names, for both a real-but-unqueried element and one missing from the SpatialData object entirely (reachable e.g. via sdata.subset(..., filter_tables=False)). "left", "left_exclusive", "inner" and "right_exclusive" all scope themselves to the queried elements only; "right" always returns the table unfiltered. So "right_exclusive" isn't simply "right minus inner" here, and the two unqueried-region scenarios are currently indistinguishable to join_spatialelement_table.

No behavior change; this documents the status quo ahead of a design decision on which of these two behaviors (if either) is intended.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.89%. Comparing base (8def24d) to head (8e85e13).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1195   +/-   ##
=======================================
  Coverage   91.89%   91.89%           
=======================================
  Files          53       53           
  Lines        7942     7942           
=======================================
  Hits         7298     7298           
  Misses        644      644           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.