add mask_3D_frac_approx to api docs

[mathause]

This was forgotten when I added the method.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.85%. Comparing base (e74cb22) to head (0be7265).
Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #534      +/-   ##
==========================================
+ Coverage   99.79%   99.85%   +0.06%     
==========================================
  Files          17       17              
  Lines        1435     1407      -28     
==========================================
- Hits         1432     1405      -27     
+ Misses          3        2       -1     

Flag	Coverage Δ
bare-minimum	?
min-all-deps	?
shapely_lt_2	?
unittests	99.85% <ø> (+0.06%)	⬆️

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