docs: updated gotchas section

doc/28_ref_optional_semantics.qbk

@@ -770,6 +770,7 @@ __SPACE__
 [: `template<class R> optional<T&>::optional(R&& r) noexcept;`]
 * [*Postconditions:] `bool(*this) == true`; `addressof(**this) == addressof(r)`.
 * [*Remarks:] Unless `R` is an lvalue reference, the program is ill-formed. This constructor does not participate in overload resolution if `decay<R>` is an instance of `boost::optional`.
+* [*Notes:] This constructor is declared `explicit` on compilers that do not correctly suport binding to const lvalues of integral types. For more details [link optional_reference_binding see here]. 
 * [*Example:]
 ``
 T v;