Clean up documentation in the create_phantom_invoice functions

[Kurtsley]

Removed the statement that belongs in the changelog.
Added the correct syntax to duration_since_epoch.

[codecov-commenter]

Codecov Report

Base: 90.85% // Head: 90.88% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (342741a) compared to base (8ecd7c3).
Patch has no changes to coverable lines.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1997      +/-   ##
==========================================
+ Coverage   90.85%   90.88%   +0.02%     
==========================================
  Files          99       99              
  Lines       52570    52570              
  Branches    52570    52570              
==========================================
+ Hits        47763    47776      +13     
+ Misses       4807     4794      -13     

Impacted Files	Coverage Δ
lightning-invoice/src/utils.rs	97.62% <ø> (ø)
lightning/src/ln/monitor_tests.rs	99.45% <0.00%> (-0.11%)	⬇️
lightning/src/ln/channelmanager.rs	87.25% <0.00%> (-0.03%)	⬇️
lightning/src/ln/functional_tests.rs	96.87% <0.00%> (+0.24%)	⬆️

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