Update documentation to correctly reference under_cached_property

[meanmail]

What do these changes do?

This PR updates the documentation in docs/api.rst to correctly reference under_cached_property instead of cached_property. The change ensures consistency and clarity in describing how the decorator works.

Are there changes in behavior for the user?

No functional changes; only documentation updates.

Related issue number

N/A

Checklist

• I think the code is well written
• Unit tests for the changes exist (not applicable, documentation-only change)
• Documentation reflects the changes

[codspeed-hq]

CodSpeed Performance Report

Merging #109 will not alter performance

_{Comparing meanmail:patch-1 (5a122b8) with master (3c03ef0)}

Summary

✅ 4 untouched benchmarks

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.76%. Comparing base (79088e6) to head (5a122b8).
⚠️ Report is 26 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #109      +/-   ##
==========================================
+ Coverage   97.72%   97.76%   +0.03%     
==========================================
  Files          18       17       -1     
  Lines         792      760      -32     
  Branches       41       41              
==========================================
- Hits          774      743      -31     
+ Misses         10        9       -1     
  Partials        8        8              

Flag	Coverage Δ
CI-GHA	97.76% <ø> (+0.03%)	⬆️
MyPy	93.30% <ø> (+0.17%)	⬆️
OS-Linux	96.91% <ø> (-0.29%)	⬇️
OS-Windows	93.27% <ø> (ø)
OS-macOS	93.27% <ø> (-0.64%)	⬇️
Py-3.10.11	91.03% <ø> (-0.85%)	⬇️
Py-3.10.16	?
Py-3.10.17	93.27% <ø> (?)
Py-3.11.11	?
Py-3.11.12	95.23% <ø> (?)
Py-3.11.9	92.99% <ø> (-0.66%)	⬇️
Py-3.12.10	95.23% <ø> (?)
Py-3.12.9	?
Py-3.13.1	?
Py-3.13.2	?
Py-3.13.3	95.23% <ø> (?)
Py-3.13.4t	95.23% <ø> (?)
Py-3.9.13	91.01% <ø> (-0.85%)	⬇️
Py-3.9.21	?
Py-3.9.22	93.25% <ø> (?)
Py-pypy7.3.16	92.97% <ø> (ø)
Py-pypy7.3.19	92.99% <ø> (ø)
VM-macos-latest	93.27% <ø> (-0.64%)	⬇️
VM-ubuntu-latest	96.91% <ø> (-0.29%)	⬇️
VM-windows-latest	93.27% <ø> (ø)
pytest	96.91% <ø> (-0.29%)	⬇️

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