Fix: prevented the binding illegal temporary to optional<const int&>

Older MSVC versions add illegal temporary when you want to assign from const integral value.

doc/11_development.qbk

@@ -228,7 +228,7 @@ For value access operations `optional<>` uses operators `*` and `->` to
 lexically warn about the possibly uninitialized state appealing to the
 familiar pointer semantics w.r.t. to null pointers.
 
-[warning
+[caution
 However, it is particularly important to note that `optional<>` objects
 are not pointers. [_`optional<>` is not, and does not model, a pointer].
 ]