Add a note to AllocRef, that implementation don't always has to be 'static

[TimDiekmann]

In rust-lang/rust#79327 some 'static bounds were added to the Pin-API of Box. A note should be added somewhere (most likely to AllocRef, that this is not always required.

as we add allocator parameters, it can be easy to think that it'll always be 'static. Noting somewhere that this is not true explicitly I think would be good. Not directly related to Pin, though.

^{This is a reminder for me so that I don't forget to add it as soon as I have a chance.}