specs: clarify NSID "fragment" and Lexicon name syntax; and globbing

[bnewbold]

In the current NSID spec, we say:

The fragment identifier string (after the #) has the same syntax restrictions as the final segment of an NSID: ASCII alphabetic, one or more characters, length restricted, etc.

Should probably review and confirm the specifics: underscores? dashes? and cross-reference these restrictions from the Lexicon spec.

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Also in the same document we say:

For example, com.atproto.* would refer to any NSIDs under the atproto.com domain authority, including nested sub-domains (sub-authorities).

We may want to change tack on this and make the glob only apply to the same "group" (aka, not recursive). And maybe introduce/define the "group" terminology.