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.

library/core/src/hash/mod.rs

@@ -164,6 +164,19 @@ mod sip;
 /// `0xFF` byte to the `Hasher` so that the values `("ab", "c")` and `("a",
 /// "bc")` hash differently.
 ///
+/// ## Portability
+///
+/// Due to differences in endianness and type sizes data fed by `Hash` to a `Hasher`
+/// should not be considered portable across platforms. Additionally the data passed by most
+/// standard library types should not be considered stable between compiler versions.
+///
+/// This means tests shouldn't probe hard-coded hash values or data fed to a `Hasher` and
+/// instead should check consistency with `Eq`.
+///
+/// Serialization formats intended to he portable between platforms or compiler versions should
+/// either avoid encoding hashes or only rely on `Hash` and `Hasher` implementations that
+/// provide additional guarantees.
+///
 /// [`HashMap`]: ../../std/collections/struct.HashMap.html
 /// [`HashSet`]: ../../std/collections/struct.HashSet.html
 /// [`hash`]: Hash::hash