Add a comment about NaN in min/max extensions

[slovnicki]

I noticed that NaN will be both min and max, which might be confusing to the user if not documented.

I do think returning NaNs is better approach than the other 2 alternatives I can think of:

• Ignoring them, which will not let user know they have NaNs in the iterable.
• Relying on false from comparisons to NaN, which will lead to inconsistent returns depending on NaN's placement (e.g. first or not first) in the iterable.

[lrhn]

Hmm, pretty over-engineered code, now that I look at it again.

  num? get minOrNull {
    var empty = true;
    var min = double.infinity;
    for (var value in this) {
       empty = false;
       if (value.isNaN) return value;
       if (value < min) min = value;
    }
    if (empty) return null;
    return min;
  }

seems much more optimizable.

(Maybe I should make a benchmark 😁)

[lrhn]

LGTM with the different phrasing.

[lrhn]

Looks good to me. Thanks!