Document root causes of parser test failures #3610

Job	Run time
Test with MIME database	1m 55s
Test x86_64-unknown-linux-gnu	2m 13s
Output Regression Test	2m 30s
Test aarch64-unknown-linux-gnu	3m 19s
Test x86_64-pc-windows-msvc	2m 23s
Test aarch64-pc-windows-msvc	2m 47s
Test aarch64-apple-darwin	1m 53s
Check Linux Packaging	2m 0s
Test Linux (latest stable Rust)	1m 54s
Test x86_64-unknown-linux-musl	2m 46s
Test x86_64-apple-darwin	6m 24s
Typo Check	10s
Actionlint	14s
Rustfmt	19s
	30m 47s

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