docs: replace types mixed at Validation\StrictRules\Rule.

[ping-yee]

Description
See #6310

If there is if (! is_string($value)) judgement, I chagned the type mixed to array|bool|float|int|object|string|null.
If there isn't the judgement of non-string, I changed the type string|null.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[kenjis]

If there isn't the judgement of non-string, I changed the type string|null.

I don't understand why.
Attackers can send non-string values as JSON.

[ping-yee]

I got it.
I'll change that.

[paulbalandan]

Thank you, @ping-yee