Yes, you can. It means we don't use 'a' (or 'the') with possessive pronouns - my, your, his, her, etc.

Yes, you can say 'a boy's toy'... The apostrophe indicates possession, but it is not in itself "a possessive". The possessive referred to in the blue box (my) is a possessive adjective.

Yes, what the screenshot means is that you can't say: A his toy.