documentation of query strategy return

[zhangyu94]

Hi, the documented return of query strategies seems inconsistent with the current implementation.

For example, the actual return of SVM_binary_minimum is the query index. https://github.com/modAL-python/modAL/blob/99d4947f69f0b2247f9916a6ef44463f7af06c55/modAL/multilabel.py#L45-L46

But in the code comment and documentation website, it is said to be a tuple of the query index and the queried instance.

https://github.com/modAL-python/modAL/blob/99d4947f69f0b2247f9916a6ef44463f7af06c55/modAL/multilabel.py#L59-L62