Restructure the storage page into search and provenance, duplicate ids,
ranking, Python operations and CLI commands, and state each in reference
voice. `Several Databases` becomes `Multiple Databases`, with the anchor
carried through every referrer, and the instruction files follow the same
name. Example databases are `papers`, `wiki` and `notes`.
`AmbiguousCitationError` is raised for a cited chunk id held by more than one
selected database, not for any shared id.
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