Clarify that nan is supported in zero_division

[robmarkcole]

This option added in #2198 to create equivalence with sklearn also supports passing through a nan if I am not mistaken, based on the implementation of torchmetrics.utilities.compute._safe_divide.
I wanted to clarify that nan can indeed be passed through, ask if any code changed are required to support this officially, and suggest this could be added to the docs which state currently zero_division [float] – Should be 0 or 1., e.g. here

[github-actions]

Hi! thanks for your contribution!, great first issue!