Fix new `clippy::doc_markdown` warnings. #664

Job	Run time
Lint	6m 7s
cargo-deny	59s
Android (ubuntu-24.04, aarch64-linux-android)	8m 15s
Difftest (windows-2022)	17m 19s
Compiletest (ubuntu-24.04)	8m 19s
Test (macOS-latest)	12m 22s
Compiletest (windows-2022)	15m 45s
Difftest (ubuntu-24.04)	9m 23s
Difftest (macOS-latest)	9m 58s
Compiletest (macOS-latest)	10m 7s
Test (ubuntu-24.04)	7m 59s
Test (windows-2022)	14m 35s
test_success	2s
	2h 1m 10s

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