Clarify na_color docstring for groups= filtering behavior

[timtreis]

Fixes #600.

Summary

• The old wording default "default" (gets set to "lightgray") implied that na_color="lightgray" was a no-op, but any explicit color enables the "show non-matching elements" path
• Updated all three render functions (render_shapes, render_points, render_labels) to clearly state: omitting na_color hides non-matching elements; passing any explicit color shows them in that color
• No code change — the behavior is correct and intentional

[timtreis]

Closing to reopen with a clean branch.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.36%. Comparing base (f3e13b8) to head (5e3eb40).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #639      +/-   ##
==========================================
- Coverage   76.39%   76.36%   -0.04%     
==========================================
  Files          11       11              
  Lines        3237     3237              
  Branches      759      759              
==========================================
- Hits         2473     2472       -1     
  Misses        466      466              
- Partials      298      299       +1     

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/basic.py	85.48% <ø> (ø)

... and 1 file with indirect coverage changes

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