Clarify overlapping accesses in Atomic*::from_ptr docs

library/core/src/sync/atomic.rs

@@ -357,8 +357,11 @@ impl AtomicBool {
     ///     duration of lifetime `'a`. Most use cases should be able to follow this guideline.
     ///   * This requirement is also trivially satisfied if all accesses (atomic or not) are done
     ///     from the same thread.
-    /// * This method must not be used to perform overlapping or mixed-size atomic
-    ///   accesses, as these are not supported by the memory model.
+    /// * Atomic accesses that overlap must either all use the same address and size,
+    ///   or they must have a happens-before relationship.
+    ///   * In other words, within one of the atomic time periods mentioned in the previous
+    ///      bullet, there cannot be accesses to the same value that overlap only partially.
+    ///      They must either not overlap at all or overlap perfectly.
     ///
     /// [valid]: crate::ptr#safety
     #[stable(feature = "atomic_from_ptr", since = "CURRENT_RUSTC_VERSION")]
@@ -1161,8 +1164,11 @@ impl<T> AtomicPtr<T> {
     ///     duration of lifetime `'a`. Most use cases should be able to follow this guideline.
     ///   * This requirement is also trivially satisfied if all accesses (atomic or not) are done
     ///     from the same thread.
-    /// * This method must not be used to perform overlapping or mixed-size atomic accesses, as
-    ///   these are not supported by the memory model.
+    /// * Atomic accesses that overlap must either all use the same address and size,
+    ///   or they must have a happens-before relationship.
+    ///   * In other words, within one of the atomic time periods mentioned in the previous
+    ///      bullet, there cannot be accesses to the same value that overlap only partially.
+    ///      They must either not overlap at all or overlap perfectly.
     ///
     /// [valid]: crate::ptr#safety
     #[stable(feature = "atomic_from_ptr", since = "CURRENT_RUSTC_VERSION")]
@@ -2144,8 +2150,11 @@ macro_rules! atomic_int {
             ///     this guideline.
             ///   * This requirement is also trivially satisfied if all accesses (atomic or not) are
             ///     done from the same thread.
-            /// * This method must not be used to perform overlapping or mixed-size atomic
-            ///   accesses, as these are not supported by the memory model.
+            /// * Atomic accesses that overlap must either all use the same address and size,
+            ///   or they must have a happens-before relationship.
+            ///   * In other words, within one of the atomic time periods mentioned in the previous
+            ///      bullet, there cannot be accesses to the same value that overlap only partially.
+            ///      They must either not overlap at all or overlap perfectly.
             ///
             /// [valid]: crate::ptr#safety
             #[stable(feature = "atomic_from_ptr", since = "CURRENT_RUSTC_VERSION")]