lexicon: document PDS validation behavior

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PDS has three Lexicon validation options when creating (or updating) records:

• explicit validation required: the record must be validated against a Lexicon schema. If the PDS does not know of the Lexicon, and/or can not resolve it on demand, the record creation fails
• explicit no validation: the record is not validated against any Lexicon, even if the PDS knows the Lexicon locally. the record still has to pass data validation rules (eg, no malformed blob objects at any nested depth)
• optimistic validation (default if no explicit argument): if the PDS knows the record Lexicon, it validates. but if it does not know it, and can't resolve it (or does not do live validation), the record is allowed to be created. aka, "fail open".

In success responses, where the record was persisted in the repository, a flag indicates if Lexicon validation happened or not.

This behavior should be added to the Lexicon specs.