Update docs for ChannelDetails fields which change on splice

[TheBlueMatt]

Various fields in ChannelDetails refer to channel information which changes on splice, which we ensure is consistently documented here.

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[benthecarman]

lgtm

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.32%. Comparing base (fe5d942) to head (c3cc331).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4250   +/-   ##
=======================================
  Coverage   89.32%   89.32%           
=======================================
  Files         180      180           
  Lines      138730   138730           
  Branches   138730   138730           
=======================================
+ Hits       123914   123922    +8     
+ Misses      12190    12184    -6     
+ Partials     2626     2624    -2     

Flag	Coverage Δ
fuzzing	35.02% <ø> (ø)
tests	88.69% <ø> (+<0.01%)	⬆️

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[tnull]

Now adopted for LDK Node's variant of ChannelDetails over at lightningdevkit/ldk-node#721

[TheBlueMatt]

Backported in #4254