docs: [Migration] clarify that $db is for testing purposes

[kenjis]

Description
Related #8221

• clarify that $db is for testing purposes

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

[MGatner]

Interesting that @internal can be used inline: https://docs.phpdoc.org/2.9/references/phpdoc/inline-tags/internal.html

I wonder if any static analyzers would enforce this on a parameter?

Suggested change

	* @param array|ConnectionInterface|string|null $db DB group. For testing purposes only.
	* @param array|ConnectionInterface|string|null $db DB group. {@internal For testing purposes only.}