docs: fix outdated statements in the SpatialData class docstring

[jan-glx]

Fixes several outdated statements in the SpatialData class docstring (src/spatialdata/_core/spatialdata.py), one commit each:

1. table → tables: the parameter was renamed to tables (a dict of AnnData) but the docstring still documented table. Also documents the previously undocumented attrs parameter.
2. Tables can annotate points: the docstring said points "can't be annotated by a table", but tables can annotate points just like labels and shapes (see docs/design_doc.md and the join/query code).
3. datatree.DataTree → xarray.DataTree: matches the type actually used (spatialdata._types.Raster_T).

Still open: the line saying tables store "the spatial coordinates ... in the obsm slot". this is covered in a separate PR #1274 that might need some discussion.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.42%. Comparing base (560caf9) to head (913faf0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1273   +/-   ##
=======================================
  Coverage   91.42%   91.42%           
=======================================
  Files          53       53           
  Lines        8381     8381           
=======================================
  Hits         7662     7662           
  Misses        719      719           

Files with missing lines	Coverage Δ
src/spatialdata/_core/spatialdata.py	93.46% <ø> (ø)

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