Documents the behavior added in datajoint/datajoint-python#1526 (closes #1512), and flips the now-obsolete guidance from #227.

Changes

• reference/specs/table-declaration.md §2.6 (new) — Foreign-Key Indexes. Normative statement of the guarantee: FK columns are indexed on both backends — implicit on MySQL/InnoDB, emitted by DataJoint on PostgreSQL — and the emitted index is coverage-aware (skipped when the columns are already a left-prefix of the PK, a declared index, or a wider FK index). Notes that lifecycle on FK drop / alter() is out of declaration scope.
• reference/specs/database-backends.md §Backend-Specific Behavior. Adds a "Foreign-key column index" row (Implicit / Emitted by DataJoint) with a pointer to the declaration spec — the difference is only who creates the index.
• explanation/referential-integrity.md §5. Flips the guidance added in docs: add Data Integrity, Referential Integrity, and Transactions explanation pages #227: it previously told users to declare an explicit index(...) on PostgreSQL; that index is now created by the framework, so the mental model "the database indexes foreign-key columns" holds on both backends.

Why now (not the spec-reshape)

§5's old text becomes actively wrong the moment #1526 merges (it tells users to hand-declare an index the framework now creates), so this shouldn't wait for the larger diagram-operations spec reshape.