docs(timeline): clarify what mark_as_read actually does

[Johennes]

These methods appear to do something very different from what the doc comment currently says.

• Public API changes documented in changelogs (optional)

[codspeed-hq]

CodSpeed Performance Report

Merging #5906 will not alter performance

_{Comparing Johennes:johannes/mark-as-read-docs (885cceb) with main (24d7518)}

Summary

✅ 50 untouched

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.66%. Comparing base (24d7518) to head (885cceb).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5906   +/-   ##
=======================================
  Coverage   88.65%   88.66%           
=======================================
  Files         362      362           
  Lines      104365   104365           
  Branches   104365   104365           
=======================================
+ Hits        92530    92536    +6     
+ Misses       7506     7498    -8     
- Partials     4329     4331    +2     

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