DocClarity: Chainsync ChannelManager before any operation

[G8XSU]

As part of #1792

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.37%. Comparing base (1d2a27d) to head (baa8ec8).

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2994      +/-   ##
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- Coverage   89.40%   89.37%   -0.03%     
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  Files         117      117              
  Lines       96016    96016              
  Branches    96016    96016              
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- Hits        85842    85818      -24     
- Misses       7957     7978      +21     
- Partials     2217     2220       +3     

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[TheBlueMatt]

Is this unclear in the ChannelManagerReadArgs documentation? The sentence prior references seeing the ChannelManagerReadArgs as the authoritative source for what you need to do, and just including one of the several things in there doesn't seem like its adding a lot of clarity.

[G8XSU]

I agree that ChannelManagerReadArgs is authoritative source,
earlier i thought it only mentions channelManager chainsync, but I noticed it mentions both channelmanager and channelMonitor to be synced.

There is also a note on "must occur before you call any other methods on the newly-deserialized [ChannelManager]"

I am good with closing this PR as redundant.