std.array_list: add a comment on every methods potentially invalidating pointers.

[remeh]

While it is already mentioned on the items attributes of the structs, it is interesting to comment in every method potentially invalidating pointers to items that they may do so. My thoughts are that it's the moment you're calling these methods that it is interesting to know you may have an impact on items pointers (I've been bitten by this...).

Please discard if you don't think that's a good idea.