Cleanup comments and docs in channel fee update logic

[TheBlueMatt]

Following up on @ariard's review at #985 (review)

[codecov]

Codecov Report

Merging #1047 (623da4d) into main (a369f9e) will increase coverage by 1.37%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main    #1047      +/-   ##
==========================================
+ Coverage   90.96%   92.33%   +1.37%     
==========================================
  Files          64       65       +1     
  Lines       32393    43186   +10793     
==========================================
+ Hits        29467    39877   +10410     
- Misses       2926     3309     +383     

Impacted Files	Coverage Δ
lightning/src/ln/channel.rs	92.66% <50.00%> (+3.29%)	⬆️
lightning/src/util/ser_macros.rs	90.52% <0.00%> (-5.79%)	⬇️
lightning/src/util/enforcing_trait_impls.rs	89.54% <0.00%> (-0.85%)	⬇️
lightning/src/ln/script.rs	93.57% <0.00%> (-0.18%)	⬇️
lightning-invoice/src/de.rs	80.62% <0.00%> (-0.12%)	⬇️
lightning/src/ln/shutdown_tests.rs	95.58% <0.00%> (ø)
lightning/src/ln/chanmon_update_fail_tests.rs	97.90% <0.00%> (+0.03%)	⬆️
lightning/src/ln/features.rs	99.60% <0.00%> (+0.16%)	⬆️
lightning-background-processor/src/lib.rs	94.17% <0.00%> (+0.23%)	⬆️
lightning/src/ln/chan_utils.rs	98.37% <0.00%> (+0.94%)	⬆️
... and 19 more

---

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[ariard]

ACK dd3097c-modulo-if-you-feel-it : #985 (comment)

[ariard]

ACK ce4e20d

[TheBlueMatt]

Reverted the additional debug_assert as it doesn't pass borrowck, which presumably is why it wasnt in the first pr either :)

[ariard]

re-ACK dd3097c

[valentinewallace]

Otherwise ACK

[TheBlueMatt]

Only change from ariard's ack is comment fixups suggested by val, will take after CI:

$ git diff-tree -U1 dd3097c 623da4da
diff --git a/lightning/src/ln/channel.rs b/lightning/src/ln/channel.rs
index b0f733454..f8cbc26a2 100644
--- a/lightning/src/ln/channel.rs
+++ b/lightning/src/ln/channel.rs
@@ -433,7 +433,7 @@ pub(super) struct Channel<Signer: Sign> {
 	pending_update_fee: Option<(u32, FeeUpdateState)>,
-	// If an update_fee() call is made with ChannelState::AwaitingRemoteRevoke set, we place it
-	// here instead of `pending_update_fee` in the same way as we place outbound HTLC updates in
+	// If a `send_update_fee()` call is made with ChannelState::AwaitingRemoteRevoke set, we place
+	// it here instead of `pending_update_fee` in the same way as we place outbound HTLC updates in
 	// `holding_cell_htlc_updates` instead of `pending_outbound_htlcs`. It is released into
 	// `pending_update_fee` with the same criteria as outbound HTLC updates but can be updated by
-	// further `update_fee` calls, dropping the previous holding cell update entirely.
+	// further `send_update_fee` calls, dropping the previous holding cell update entirely.
 	holding_cell_update_fee: Option<u32>,
$