chore(ui): Clarifies what TimelineItemPosition::UpdateDecrypted holds

[Hywan]

(Extracted from #3512)

This patch tries to clear confusion around
TimelineItemPosition::UpdateDecrypted(usize): it does contains a timeline item index. This patch changes to
TimelineItemPosition::UpdateDecrypted { timeline_item_index: usize }

---

• Address [meta] EventCache storage #3280

[codecov]

Codecov Report

Attention: Patch coverage is 54.54545% with 5 lines in your changes missing coverage. Please review.

Project coverage is 85.07%. Comparing base (75d7d07) to head (2d8b6bd).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/matrix-sdk-ui/src/timeline/event_handler.rs	50.00%	4 Missing ⚠️
...tes/matrix-sdk-ui/src/timeline/controller/state.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4330      +/-   ##
==========================================
+ Coverage   85.05%   85.07%   +0.02%     
==========================================
  Files         275      275              
  Lines       30309    30312       +3     
==========================================
+ Hits        25780    25789       +9     
+ Misses       4529     4523       -6     

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[andybalaam]

Strong recommend!