Fixed #29762 -- Document how database routers are used for related-object access.

[VIZZARD-X]

Trac ticket number

ticket-29762

Branch description

This PR documents how Django selects the database for related-object access in multi-database configurations.
Django records the originating database on instance._state.db and uses it for related lookups.
When a database router overrides db_for_read() without considering the instance hint, this default behavior is lost, which can cause errors in multi-schema setups.
The patch adds a section explaining the default behavior, the use of hints, and how routers should preserve correct routing.

Checklist

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