Add documentation for [[msvc::musttail]] attribute

Document the [[msvc::musttail]] attribute with constraints and an example.

Author: HulonJenkins

docs/cpp/attributes.md

@@ -158,6 +158,41 @@ void f() {
     i = my_move(i);
 }
 ```
+### `[[msvc::musttail]]`
+
+The `[[msvc::musttail]]` attribute is an experimental x64 only Microsoft-specific attribute that enforces tail-call optimization for a given function call. When applied on a qualifying return statement, it instructs the compiler to emit the call as a tail call. If the compiler cannot do so, it will produce a compilation error. The `[[msvc::musttail]]` attribute will enforce tail calling over inlining of a function.
+
+The `[[msvc::musttail]]` attribute has constraints:
+- The caller and callee must have matching return types.
+- The calling conventions must be compatible.
+- The tail call must be in the return position (i.e., the final action in the calling function).
+- The callee must not use more stack space than the calling function.
+- If more than 4 integer parameters are passed, the calling function must allocate enough stack space for those additional arguments. Please see __preserve_none calling convention for tail calling without additional stack overhead.
+- Other compiler optimizations must be enabled.
+
+#### Example
+
+In this sample code, the `[[msvc::musttail]]` attribute applied to the return bar function call forces execution to jump from foo to bar before returning to main, rather than inlining bar into foo or calling bar from foo and returning to foo before returning to main.
+
+```cpp
+int bar(int x) {
+    return x + 1;
+}
+
+int foo(int x) {
+    if(x > 0){
+	[[msvc::musttail]]
+return bar(x);
+}
+return -1;
+}
+
+int main() {
+    int result = foo(42);
+    if(result < 0) return -1;
+    return 0;
+}
+```
 
 ### `[[msvc::noinline]]`