Describe valid flake ids in nix3-flake man page #13061
Description
Problem
I do not know which flake ids are valid to use for indirect flake references, to be mapped by the nix registry.
Proposal
Add such a definition to the general overview on flake references,
e.g. to nix3-flake(1).
E.g. state that [A-Za-z]+ is valid (not checked).
Checklist
- checked latest Nix manual (source)
- checked open documentation issues and pull requests for possible duplicates
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