Clarify provenance and reborrowing

Author: joshlf

library/core/src/mem/maybe_uninit.rs

@@ -284,6 +284,12 @@ use crate::{fmt, intrinsics, ptr, slice};
 /// and back will always produce the original value `t` again. This is true even if `t` contains [provenance]:
 /// the resulting value will have the same provenance as the original `t`.
 ///
+/// Note a potential footgun: if `t` contains a reference, then there may be implicit reborrows of the
+/// reference any time it is copied, which may alter its provenance. In that case, while the value returned
+/// by `identity` is exactly the same as its argument, that value may immediately be reborrowed upon return,
+/// altering its provenance. This may make this call to `identity` behave as though it does *not* exactly
+/// preserve provenance.
+///
 /// [provenance]: crate::ptr#provenance
 ///
 /// [reference-byte]: ../../reference/memory-model.html#bytes