Fix Clippy issues in `register-docs` gated code #1688

Job	Run time
clippy	1m 41s
doc-lints	45s
miri-test	1m 38s
godot-itest (macos-x86)	3m 6s
proptest	27s
run-examples	1m 26s
godot-itest (macos-double-x86)	4m 23s
rustfmt	24s
godot-itest (macos-x86-4.2)	1m 52s
godot-itest (macos-arm)	1m 29s
unit-test (macos)	1m 14s
cargo-deny-machete	10s
godot-itest (macos-arm-4.2)	1m 4s
unit-test (windows)	3m 3s
godot-itest (windows)	3m 49s
unit-test (linux)	1m 29s
license-guard	33s
unit-test (linux-minimal-deps)	1m 24s
godot-itest (windows-double)	3m 49s
unit-test (linux-msrv)	1m 30s
godot-itest (windows-4.2)	1m 54s
godot-itest (linux-full)	2m 32s
godot-itest (linux-double-lazy)	2m 11s
godot-itest (linux-features-experimental)	2m 7s
godot-itest (linux-release)	1m 7s
godot-itest (linux-4.3)	1m 34s
godot-itest (linux-4.2)	1m 37s
godot-itest (linux-memcheck-nightly)	2m 37s
godot-itest (linux-memcheck-4.1)	1m 47s
ci-status	0s
	52m 42s

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