Correction to the documentation of function for `HashMap::compute`

[MathieuDutSik]

One requirement for the function HashMap::compute is that the function are "pure" because they can be called several times.
However, that seems too strong.
Instead, the right condition should be that the function is idempotent, that is returns the same result when called several times.

[ibraheemdev]

That is true, though the point of the documentation is to highlight that there is no guarantee as to how many times the function is executed, so side effects may lead to confusing results.