diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -636,7 +636,7 @@
 def AlwaysInline : InheritableAttr {
   let Spellings = [GCC<"always_inline">, Keyword<"__forceinline">];
   let Subjects = SubjectList<[Function]>;
-  let Documentation = [Undocumented];
+  let Documentation = [AlwaysInlineDocs];
 }
 
 def Artificial : InheritableAttr {
diff --git a/clang/include/clang/Basic/AttrDocs.td b/clang/include/clang/Basic/AttrDocs.td
--- a/clang/include/clang/Basic/AttrDocs.td
+++ b/clang/include/clang/Basic/AttrDocs.td
@@ -4391,3 +4391,22 @@
 
 }];
 }
+
+def AlwaysInlineDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+Inline heuristics are disabled and inlining is always attempted regardless of
+optimization level.
+
+Does not guarantee that inline substitution actually occurs.
+
+See also `the MSDN Inline docs`_, `the GCC Common Function Attribute docs`_,
+and `the GCC Inline docs`_.
+
+.. _the MSDN Inline docs: https://docs.microsoft.com/en-us/cpp/cpp/inline-functions-cpp
+.. _the GCC Common Function Attribute docs: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html
+.. _the GCC Inline docs: https://gcc.gnu.org/onlinedocs/gcc/Inline.html
+
+}];
+  let Heading = "always_inline, __force_inline";
+}