doc: don't use typehints for signatures.

[jakevdp]

The reason to avoid this is that overloaded signatures can be very confusing.

This is the current documentation page for jax.numpy.where:

This is the same page built with this PR:

The downside is that for non-overloaded functions, this change causes type annotations to not be inserted into parameter descriptions. For example this is the current documentation page for jax.scipy.linalg.inv:

And this is the same page built with this PR:

I think the benefit in the case of jnp.where and other overloaded functions far outweighs the loss of annotations in other cases.