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 

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