Minor documentation fix for dapr python actor sdk mock actor capability

[lor1113]

Description

Added a short one liner to the python actor SDK docks explaining the limitation referenced in the issue.

Issue reference

We strive to have all PR being opened based on an issue, where the problem or feature have been discussed prior to implementation.

Please reference the issue this PR will close: #800 - It will not close the issue. I will close the issue once I have actually implemented full mocking of this, or determined that it is impossible. Nonetheless it's obviously worth updating the documentation first.

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• [No code changes] Code compiles correctly
• [No code changes] Created/updated tests
• Extended the documentation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.39%. Comparing base (bffb749) to head (db073a7).
Report is 26 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #801      +/-   ##
==========================================
- Coverage   86.63%   86.39%   -0.24%     
==========================================
  Files          84       89       +5     
  Lines        4473     5102     +629     
==========================================
+ Hits         3875     4408     +533     
- Misses        598      694      +96     

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