docs: Document that group topic lock is default on.

[iphydf]

This change is 

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 73.05%. Comparing base (9e9ed77) to head (7df3f99).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2744 +/- ## ========================================== + Coverage 72.99% 73.05% +0.05% ========================================== Files 149 149 Lines 30533 30533 ========================================== + Hits 22287 22305 +18 + Misses 8246 8228 -18 ```

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