DOC: document the typing conventions for generic ndarray

[kwsp]

Issue with current documentation:

Currently it isn't obvious how one should type ndarray: should one type them like ndarray[shape, dtype] or ndarray[dtype, shape]?

In the few places in the docs where I think this is relevant, no information is given as to how one should type ndarray:

• numpy.ndarray.class_getitem
• numpy.ndarray

Both pages link to numpy.typing.NDArray, it appears that one should follow the convention of ndarray[shape, dtype]

numpy.typing.NDArray = numpy.ndarray[typing.Any, numpy.dtype[+ScalarType]]

Idea or request for content:

If that is the case, should we make this explicit in the two doc pages linked above?

I came across this issue recently when I was writing some custom serialization/deserialization functions that take into account the shape and type of ndarrays from the type annotations, and found the typing convention isn't clearly documented. Having a convention for annotation ndarray could help make the type annotations more interoperable.

[kwsp]

Relevant discussions on how to type shapes: #16544