Rollup merge of #125043 - RalfJung:ref-type-safety-invariant, r=scottmcm

reference type safety invariant docs: clarification

The old text could have been read as saying that you can call a function if these requirements are upheld, which is definitely not true as they are an underapproximation of the actual safety invariant.

I removed the part about functions relaxing the requirements via their documentation... this seems incoherent with saying that it may actually be unsound to ever temporarily violate the requirement. Furthermore, a function *cannot* just relax this for its return value, that would in general be unsound. And the part about "unsafe code in a safe function may assume these invariants are ensured of arguments passed by the caller" also interacts with relaxing things: clearly, if the invariant has been relaxed, unsafe code cannot rely on it any more. There may be a place to give general guidance on what kinds of function contracts can exist, but the reference type is definitely not the right place to write that down.

I also took a clarification from https://github.com/rust-lang/rust/pull/121965 that is orthogonal to the rest of that PR.

Cc ```@joshlf``` ```@scottmcm```

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