[Docs] LSPS2: Improve invoice_parameters_generated docs

[martinsaposnic]

Closes #3478

On invoice_parameters_generated, explicitly mention that the intercept SCID needs to be retrieved from ChannelManager::get_intercept_scid

This PR documents and matches exactly what ldk-node is doing here https://github.com/lightningdevkit/ldk-node/blob/main/src/liquidity.rs#L554-L586

[ldk-reviews-bot]

I've assigned @jkczyz as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.73%. Comparing base (b5bfa85) to head (5d2f8bc).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3871      +/-   ##
==========================================
- Coverage   89.75%   89.73%   -0.03%     
==========================================
  Files         164      164              
  Lines      132834   133079     +245     
  Branches   132834   133079     +245     
==========================================
+ Hits       119229   119413     +184     
- Misses      10926    10978      +52     
- Partials     2679     2688       +9     

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