If that documented behavior is part of the API contract, then the entire isValid function seems useless and should probably be removed – if it's intended to check emptiness, just use isEmpty which already does that (or rename isValid to isNotEmpty).

uh, no, I don't agree with your point. Your example is turning a bugfix into a breaking change by describing the implementation details of a method its documentation, which becomes inaccurate after changing the implementation.

Good developers solve this by not describing the implementation details of functions in comments. This is commonly known as encapsulation.[0] This is why most libraries do not have comments detailing the implementation like you suggest.

[0] https://en.wikipedia.org/wiki/Encapsulation_(computer_progra...