Add a section to the module documentation "What is a legal allocator"

[TimDiekmann]

As noted in https://github.com/rust-lang/wg-allocators/issues/44#issuecomment-596904626, we probably want to add a section in the alloc-module documentation, which describes, what's an legal allocator.

As @Lokathor wrote up, this section

would be similar to how the core::ptr module has a section that describes the pointer validity rules, and how the core::sync::atomic and std::thread modules describe that Rust has some sort of system for threads and atomics and you should go read the C++ specs to learn about it because right now we just do "what llvm does" and that's "what C++ does".

The main rules would be things that "make sense anyway", like you can't start allocating from a location just past the end of the stack and then call a function because when you push stuff onto the stack it starts writing into your allocation pool and the program gets very sad.

[TimDiekmann]

This section in the RFC may be a good start.

[TimDiekmann]

Related: #65