document lf-value-json's encoding of various types

[S11001001]

This moves the specification of #2361 from draft into publicly-available documentation for users of the HTTP JSON API to refer to.

Discussion of formulation of the rules belongs in #2361; this PR is about documenting the rules that have been chosen.

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• Include appropriate tests
• Set a descriptive title and thorough description
• Add a reference to the issue this PR will solve, if appropriate
• Add a line to the release notes, if appropriate

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[leo-da]

LGTM;