Slightly Amend Zarr Encoding Specification Doc #8749

[eshort0401]

• [x] Closes #8749
• [x] User visible changes (including notable bug fixes) are documented in whats-new.rst

Slight documentation amendments following discussion in #8749 and #11006.

• I add a few lines to communicate that Xarray checks all three supported conventions; first Zarr v3, then Xarray flavoured Zarr v2, and finally NCZarr, raising an error if no dimension names are found.
• I make a few additional word choice edits for consistency and clarity, e.g. replace "Zarr arrays" with "Zarr groups" in the "Compatibility and Reading" section.
• I have also added an additional code comment to xarray.backends.zarr._get_zarr_dims_and_attrs providing the link to the (very helpful) Xarray encoding specification - I missed this document when I first looked at this code!

Additional Notes

• Discussion with @TomNicholas in #8749 mentions improving the error message given when dimension name metadata is missing, but this was implemented in #10025.
• An idea was also floated that Xarray could use default dimension names when metadata is missing, but this cannot be done unambiguously when multiple arrays are in the Zarr group, so I would consider now closing #8749.