Small update to text in section 2.3 regarding character sets about cf-conventions HOT 4 CLOSED

As Unicode contains many digits outside of 0-9, I think the term digit needs to be clarified similar to letter.

I have made a suggestion in the PR simply adding a sentence after and similar to the sentence clarifying the meaning of letter.

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OK. The ASCII "underscore" is the Unicode "low line". There is also a Unicode "combining low line" (and probably others).

So, perhaps to be really clear we should specify "underscore" as well.

By the word letters we mean the standard ASCII letters uppercase A to Z and lowercase a to z.
By the word digits we mean the standard ASCII digits 0 to 9.
By the word underscore we mean the standard ASCII underscore _.

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Thanks Ethan!

In an attempt to avoid triple repetitions I suggest a small rewording which I hope does not change the meaning. The first paragraph is then:

Variable, dimension, attribute and group names should begin with a letter and be composed of letters, digits, and underscores. By the word letters we mean the standard ASCII letters uppercase A to Z and lowercase a to z.
By the word digits we mean the standard ASCII digits 0 to 9, and similarly underscores means the standard ASCII underscore _. Note that this is in conformance with the COARDS conventions, but is more restrictive than the netCDF interface which allows use of the hyphen characteralmost all Unicode characters encoded as multibyte UTF-8 characters (NUG Appendix B). The netCDF interface also allows leading underscores in names, but the NUG states that this is reserved for system use.

I will put this in the associated PR.

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As in the PR description (above) the suggested change has already been discussed in this comment of #237 and subsequent ones. The last comment was on November 1st, and support was voiced by @ChrisBarker-NOAA.

If there are no more comments, except regarding minor technical details, I suggest that proposed changes can be implemented via PR#470 on November 22.

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