Update keysend docs + add payment secret requirement for MPP

[valentinewallace]

Addressing #967 (comment).

Closes #1026

[codecov]

Codecov Report

Merging #1038 (552a244) into main (8530078) will increase coverage by 1.57%.
The diff coverage is 97.56%.

@@            Coverage Diff             @@
##             main    #1038      +/-   ##
==========================================
+ Coverage   90.80%   92.37%   +1.57%     
==========================================
  Files          61       62       +1     
  Lines       31582    37644    +6062     
==========================================
+ Hits        28678    34774    +6096     
+ Misses       2904     2870      -34     

Impacted Files	Coverage Δ
lightning/src/ln/channelmanager.rs	89.57% <97.56%> (+3.85%)	⬆️
lightning/src/util/errors.rs	69.33% <0.00%> (-2.10%)	⬇️
lightning/src/ln/mod.rs	89.28% <0.00%> (-0.72%)	⬇️
lightning-background-processor/src/lib.rs	95.20% <0.00%> (-0.38%)	⬇️
lightning/src/ln/script.rs	93.75% <0.00%> (ø)
lightning/src/ln/chanmon_update_fail_tests.rs	97.80% <0.00%> (+0.05%)	⬆️
lightning/src/chain/keysinterface.rs	95.58% <0.00%> (+0.21%)	⬆️
lightning/src/ln/functional_test_utils.rs	95.51% <0.00%> (+0.33%)	⬆️
lightning/src/ln/functional_tests.rs	98.97% <0.00%> (+1.65%)	⬆️
lightning/src/chain/channelmonitor.rs	92.71% <0.00%> (+2.14%)	⬆️
... and 4 more

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