test: fix GPU tests and docs: fix WriteCallback param description

[flying-sheep]

In #1707, we forgot to adapt the GPU tests, so as_cupy was called twice on X (once as adata.layers[None] and once as adata.X). Since as_cupy deliberately only handles non-cupy data, this failed.

/edit: looks like some unrelated tiny docs fix also snuck in. eh, good.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.62%. Comparing base (75e4c33) to head (0035dac).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2458      +/-   ##
==========================================
+ Coverage   85.70%   87.62%   +1.92%     
==========================================
  Files          49       49              
  Lines        7806     7806              
==========================================
+ Hits         6690     6840     +150     
+ Misses       1116      966     -150     

Files with missing lines	Coverage Δ
src/anndata/_types.py	100.00% <ø> (ø)

... and 8 files with indirect coverage changes