Remove spurious docs which imply you cannot retry a failed payment

[TheBlueMatt]

If a payment (fully) fails, users are free to retry it, its only once a payment succeeds that a user must never retry it.

[wpaulino]

There's also a double comma we could fix on lines 110-111.

[codecov-commenter]

Codecov Report

Patch coverage: 97.49% and project coverage change: +0.11 🎉

Comparison is base (ede06d8) 90.33% compared to head (60ce627) 90.44%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2340      +/-   ##
==========================================
+ Coverage   90.33%   90.44%   +0.11%     
==========================================
  Files         104      104              
  Lines       53392    54046     +654     
  Branches    53392    54046     +654     
==========================================
+ Hits        48229    48881     +652     
- Misses       5163     5165       +2     

Impacted Files	Coverage Δ
lightning-invoice/src/payment.rs	88.66% <ø> (ø)
lightning/src/ln/peer_handler.rs	61.47% <89.70%> (+1.15%)	⬆️
lightning-background-processor/src/lib.rs	82.58% <100.00%> (+0.12%)	⬆️
lightning-net-tokio/src/lib.rs	76.94% <100.00%> (+0.20%)	⬆️
lightning/src/ln/chanmon_update_fail_tests.rs	98.63% <100.00%> (+0.02%)	⬆️
lightning/src/ln/channelmanager.rs	86.32% <100.00%> (+0.02%)	⬆️
lightning/src/ln/functional_test_utils.rs	88.03% <100.00%> (+0.12%)	⬆️
lightning/src/ln/functional_tests.rs	98.25% <100.00%> (+<0.01%)	⬆️
lightning/src/ln/msgs.rs	84.58% <100.00%> (+0.01%)	⬆️
lightning/src/ln/payment_tests.rs	97.58% <100.00%> (+0.01%)	⬆️
... and 7 more

... and 4 files with indirect coverage changes

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