metric.F1 doc needs clarification

[szha]

The current logic only supports what's called "macro F1" logic where we take the mean of F1 scores for mini-batches, which needs to be clarified in doc.

https://github.com/apache/incubator-mxnet/blob/master/python/mxnet/metric.py#L491-L493

[thomelane]

@szha @indhub the docs were updated as part of #9777 so this can be closed, thanks!