docs: fix docstring for Array.cdata_shape

[maxrjones]

The cdata_shape docstring states that it returns the shape of the chunk grid. However, Zarr-Python's internals use the chunk grid to model the outer boundaries (xref #3767). This changes the docstring to align with the implementation, which returns a shape for inner chunks.

This also applies to the function named _chunk_grid_shape. I updated that docstring but chose not to rename to something more technically accurate.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.md
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[maxrjones]

Or maybe this was just a bug? It's confusing why there's a .chunks and a .cdata_shape.

[d-v-b]

cdata_shape is from zarr python 2.x