Fix FAILPOINTS variable name in docs, and lints

[sourcefrog]

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name: Fix env var name in docs about: A bug squashed.

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Related bugs: n/a

Description of problem: When reading the docs I was briefly confused what the correct variable name was.1

Description of solution: Fix the docs to match the code.

Also, since rustc warns that unit tests inside docests are not run, mark it as norun for clarity and to address a warning.

Checklist: The CI will check all of these, but you'll need to have done them:

• [x] cargo fmt -- --check passes.
• [x] cargo +nightly clippy has no warnings.
• [x] cargo test passes.

[sourcefrog]

I don't know if there is any way to satisfy both stable and nightly Clippy here without allowing unknown lints, which seems like it might allow the old name to hang around, and it seems tangential to the core typo fix. Anyhow, let me know what you want, or if you're open to merging this.

[BusyJay]

Rest LGTM

[sourcefrog]

Thanks, please take another look.

[BusyJay]

Please sign off your last commit.

[sourcefrog]

Please sign off your last commit.

done