"Need not do ...": Unnecessary. "Must not do ...": Disallowed / prohibited.

TheOtherHobbes · on Dec 9, 2019

These used to be called Dos and Don'ts.

Do A, B, C. Don't do X, Y, Z.

Am I the only person who thinks it's ironic that an article about documentation appears to suffer from a fairly fundamental ambiguity?

dredmorbius · on Dec 9, 2019

No.

"Need not" (or "May...") indicate actions or behaviours which are allowed or optional. This is not the same as "do X", which is a "MUST" condition.

"Must not" (or "Don't...") indicate actions or behaviours which are expressly disallowed.

RFC 2119 referenced in the article should make all this clear. If you're still confused after reading that ... you SHOULD NOT be writing documentation. And possibly not be reading it ;-)