diff --git a/clang/docs/LanguageExtensions.rst b/clang/docs/LanguageExtensions.rst
--- a/clang/docs/LanguageExtensions.rst
+++ b/clang/docs/LanguageExtensions.rst
@@ -2454,6 +2454,14 @@
 * Implement an atomic memory with atomic operations of a particular size,
   similar to that presented in C++ proposal [p1478](https://wg21.link/p1478).
 
+When using the `_Atomic` qualifier, the memory will be accessed with a sequence
+of operations of size equal to or a multiple of the pointer's element size. The
+order of operations is unspecified, and has unordered atomic semantics. This
+means that reads and writes do not tear at the individual element level, and
+they each occur exactly once, but the order in which they occur can only be
+guaranteed using appropriate fences. Atomic memory operations must be to memory
+locations which are aligned to no less than the element size.
+
 Atomic Min/Max builtins with memory ordering
 --------------------------------------------