Clarify asserting equality of objects and arrays

Some people might expect assert.deepStrictEqual() is to be used only for nested objects, but that is not correct.

This change adds some clarification, because assert.deepStrictEqual() should be used for all objects and arrays even if they are flat.

See https://github.com/nodejs/node/issues/31263 for further reference.

Checklist

• documentation is changed or added
• commit message follows commit guidelines