docs: clarify Model::find() note for null argument

[michalsn]

Description
This PR clarifies the user guide note for Model::find() to explicitly state that calling it without arguments or with null returns all rows.

Closes #10071

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide