Add a warning to CStr::as_ptr

[Gankra]

@bluss was saying that this is a really common problem for people to run into on IRC. Hopefully this helps?

[rust-highfive]

r? @alexcrichton

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[retep998]

I agree that this is a common problem and some sort of documentation needs to be provided warning people about it.

[Gankra]

bluss noted on IRC that providing more positive/affirmitive/specific advice might be better here. As in, "Make sure the returned pointer is only used while the original CString is live". Inclined to agree, but interested in discussing the best way to handle this sort of thing.

See #27769 for some related discussion.

[alexcrichton]

Yeah I would also expect the docs to nudge you in the right direction by talking about having the original CString outlive the pointer itself

[steveklabnik]

Yeah, there should be something here. We can nitpick it a bunch, or we can just merge this, and as I go through the API docs, I can clean it up later, either is fine for now imho. (I know you're busy, @gankro )