Rollup merge of #90995 - the8472:hash-portability, r=dtolnay

Rollup merge of #90995 - the8472:hash-portability, r=dtolnay

Document non-guarantees for Hash

Dependence on endianness and type sizes was reported for enum discriminants in #74215 but it is a more general
issue since for example the default implementation of `Hasher::write_usize` uses native endianness.
Additionally the implementations of library types are occasionally changed as their internal fields
change or hashing gets optimized.

## Question

Should this go on the module level documentation instead since it also concerns `Hasher` to some extent and not just `Hash`?

resolves #74215