Update Event::PaymentReceived docs since we require payment secret

[TheBlueMatt]

Users no longer need to verify the amounts of received payments as
the payment secret will protect us against the probing attacks such
verification was intended to fix.

Reported at getlipa/ldk-sample-java#1 (comment)

[codecov]

Codecov Report

Merging #1079 (d7d7801) into main (088daf7) will decrease coverage by 0.01%.
The diff coverage is n/a.

❗ Current head d7d7801 differs from pull request most recent head 7478207. Consider uploading reports for the commit 7478207 to get more accurate results

@@            Coverage Diff             @@
##             main    #1079      +/-   ##
==========================================
- Coverage   91.01%   90.99%   -0.02%     
==========================================
  Files          65       65              
  Lines       33767    33767              
==========================================
- Hits        30732    30727       -5     
- Misses       3035     3040       +5     

Impacted Files	Coverage Δ
lightning/src/util/events.rs	27.54% <ø> (ø)
lightning/src/ln/functional_tests.rs	97.42% <0.00%> (-0.09%)	⬇️

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[TheBlueMatt]

Squashed without change, will land after CI:

$ git diff-tree -U1 d7d78012 7478207d
$