Clarify data type docs in v2 spec page

[keller-mark]

Hi, while implementing a Zarr package for R, I ran into the issue that for "U" data types, the number in the dtype string seems to be the number of characters (rather than the number of bytes as specified here).

Related, it can be clarified here that for object data types there is not a number included in the dtype string. This may be useful when writing regexes to parse the data type information out of the dtype string.

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/tutorial.rst
• [ ] Changes documented in docs/release.rst
• [ ] GitHub Actions have all passed
• [ ] Test coverage is 100% (Codecov passes)

[joshmoore]

@keller-mark: I assume this is the clarification stipulated in https://github.com/zarr-developers/zarr-specs/issues/121 ?

[jhamman]

xref https://github.com/zarr-developers/zarr-specs/pull/276