Added comment for wrenReturnBool

include/wren.h

@@ -149,6 +149,9 @@ void wrenReturnNull(WrenVM* vm);
 // [text] will be calculated using `strlen()`.
 void wrenReturnString(WrenVM* vm, const char* text, int length);
 
+// Provides a boolean return value for a foreign call. This must only be called
+// within a function provided to [wrenDefineMethod]. Once this is called, the
+// foreign call is done, and no more arguments can be read or return calls made.
 void wrenReturnBool(WrenVM* vm, bool value);
 
 #endif