(chore): add docstring example for read_elem_as_dask

[ilan-gold]

Noticed during my lab presentation, would be super helpful

• Closes #
• Tests added
• Release note added (or unnecessary): making a docstring

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.45%. Comparing base (3260222) to head (97f2755).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1722      +/-   ##
==========================================
- Coverage   86.93%   84.45%   -2.49%     
==========================================
  Files          40       40              
  Lines        6039     6039              
==========================================
- Hits         5250     5100     -150     
- Misses        789      939     +150     

Files with missing lines	Coverage Δ
src/anndata/_io/specs/registry.py	95.45% <ø> (-0.57%)	⬇️

... and 7 files with indirect coverage changes