Improve reference docs for sphereTransform

The API reference docs for `sphereTransform` (for all of Java, Matlab and Icy) currently don't make a lot of sense:

* the output is created with the same size as the input:
https://github.com/clij/clij2-docs/blob/c94f45d1bb84d9f9aa57f184d8826096e6299fe2/reference_sphereTransform.md#L35
and it's unclear how the correct output dimensions can be derived from a given input.
I'd suggest to add something along the lines of the actual computation in `clij{2,x}-assistant` to the docs:
https://github.com/clij/clij2-assistant/blob/1cef95e60735399c994c7e3186b1c2aa03059c5d/src/main/java/net/haesleinhuepf/clij2/assistant/interactive/handcrafted/SphereTransform.java#L83-L85
https://github.com/clij/clijx-assistant/blob/6b6c719098179ee3fe6afa6317aba0b6d70d927a/src/main/java/net/haesleinhuepf/clijx/assistant/interactive/handcrafted/SphereTransform.java#L83-L85

* the center coordinates are arbitrarily set:
https://github.com/clij/clij2-docs/blob/c94f45d1bb84d9f9aa57f184d8826096e6299fe2/reference_sphereTransform.md#L38-L40
If they are supposed to represent *relative* values as the name suggests, they should be between `0.0` and `1.0`, or otherwise their name should be changed (see also https://github.com/clij/clij2/issues/42)