CPP: Add an explanatory comment on WideCharType and Wchar_t.

cpp/ql/src/semmle/code/cpp/Type.qll

@@ -601,6 +601,10 @@ class VoidType extends BuiltInType {
 
 /**
  * The C/C++ wide character type.
+ * 
+ * Note that on some platforms `wchar_t` doesn't exist as a built-in
+ * type but a typedef is provided.  Consider using the `Wchar_t` QL
+ * class to include these types.
  */
 class WideCharType extends IntegralType {
 

cpp/ql/src/semmle/code/cpp/commons/CommonType.qll

@@ -80,6 +80,10 @@ class Uintmax_t extends Type {
 
 /**
  * The C/C++ wchar_t type.
+ *
+ * Note that on some platforms `wchar_t` doesn't exist as a built-in
+ * type but a typedef is provided.  This QL class includes both cases
+ * (see also `WideCharType`).
  */
 class Wchar_t extends Type {
   Wchar_t() {