Document existing database value shapes

[alexstandiford]

Summary

• document existing broad array shapes without changing native signatures
• document value shapes already guaranteed by TableSchemaService, WithDatastoreHandlerMethods, and their tests
• correct stale PHPDoc parameter names
• add legitimate project and code terms used by the current README to the spellcheck dictionary

Proof

• PHPStan level 9 diagnostics: 231 before, 177 after
• no new PHPStan diagnostics
• PHPUnit: 162 tests, 483 assertions before and after
• pyspelling: pass

This is a documentation-only cleanup. It adds no baseline, suppression, exclusion, dependency change, or runtime change. Findings that require a behavioral or contract decision remain outside this pull request.