Fix fuzzer-found panic from duplicate channel outpoint

[valentinewallace]

And update Watch docs. See added comments for more details.

Fuzz fail repro script:

export TARGET="full_stack" # adjust for your output
export HEX="010100001300000000000000000000000000000000000000000000000000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"

mkdir -p ./test_cases/$TARGET
echo $HEX | xxd -r -p > ./test_cases/$TARGET/any_filename_works

export RUSTFLAGS="--cfg=fuzzing"
export RUST_BACKTRACE=1
cargo test &> output.txt

[codecov]

Codecov Report

Merging #1080 (612d1fb) into main (b3be420) will increase coverage by 1.89%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##             main    #1080      +/-   ##
==========================================
+ Coverage   90.85%   92.75%   +1.89%     
==========================================
  Files          65       65              
  Lines       33229    43072    +9843     
==========================================
+ Hits        30191    39952    +9761     
- Misses       3038     3120      +82     

Impacted Files	Coverage Δ
lightning/src/chain/mod.rs	58.82% <ø> (ø)
lightning/src/ln/channelmanager.rs	89.33% <88.88%> (+3.49%)	⬆️
lightning/src/util/enforcing_trait_impls.rs	88.97% <0.00%> (-0.31%)	⬇️
lightning/src/util/ser_macros.rs	87.72% <0.00%> (-0.18%)	⬇️
lightning/src/ln/chanmon_update_fail_tests.rs	97.75% <0.00%> (-0.14%)	⬇️
lightning/src/ln/monitor_tests.rs	100.00% <0.00%> (ø)
lightning/src/ln/reorg_tests.rs	99.78% <0.00%> (+0.07%)	⬆️
lightning/src/ln/onion_utils.rs	95.36% <0.00%> (+0.44%)	⬆️
lightning/src/ln/chan_utils.rs	98.00% <0.00%> (+0.59%)	⬆️
lightning/src/ln/onion_route_tests.rs	98.10% <0.00%> (+0.99%)	⬆️
... and 17 more

---

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