docs: Add important disclaimer about using connectionString

[melroy89]

Add this important note, it's currently not clear unless you read your plugin code.

When using connectionString with fastify-mysql, it will NO longer pass any options towards mysql2. This could be confusing for some people.

If you however still want to use a database URI string together with additional mysql configuration options for the mysql2 package, then consider using uri option instead together with your additional options (let's say you also want to change connectionLimit as well and still use a database URI).

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm, thx