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

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.39%. Comparing base (f3e13b8) to head (16d701d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #640   +/-   ##
=======================================
  Coverage   76.39%   76.39%           
=======================================
  Files          11       11           
  Lines        3237     3237           
  Branches      759      759           
=======================================
  Hits         2473     2473           
  Misses        466      466           
  Partials      298      298           

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

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