Fixed #35434 -- Advised validating unfetched instances before testing result cache.

[jacobtylerwalls]

Trac ticket number

ticket-35434

Branch description

Clarify in the discussion of queryset caching that validating the primary key field on unfetched instances may be necessary when testing against the result cache.

Checklist

• This PR targets the main branch.
• The commit message is written in past tense, mentions the ticket number, and ends with a period.
• I have checked the "Has patch" ticket flag in the Trac system.
• I have added or updated relevant tests.
• I have added or updated relevant docs, including release notes if applicable.
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[nessita]

I'm not comfortable adding this to the docs because it could be seen as if this is a valid date field assignment when it's not (and it's my understanding that this is exactly Simon's point in the ticket). You could filter using the string but you should not do attribute assignment using any incorrect type.