Add allow-invalid configuration option for disallowed_* to the documentation

[alex-semenyuk]

Close #14836

changelog: none

[rustbot]

r? @flip1995

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[samueltardieu]

Thanks! Some nitpicking about the formatting.

[samueltardieu]

Some more formatting nitpicking, otherwise it looks good to me. I'll let @flip1995 have the final word though.

[alex-semenyuk]

@flip1995 any ideas what can be improved, thanks

[alex-semenyuk]

@rustbot r? clippy

[samueltardieu]

Thanks.

[flip1995]

Thanks!