Cleanup: doxygen comments

Also remove duplicate & mismatching comments from grease-pencil header.
Keep comments close to implementation to avoid getting out of sync.

intern/ghost/GHOST_C-api.h

@@ -43,7 +43,7 @@ extern "C" {
  * Creates a "handle" for a C++ GHOST object.
  * A handle is just an opaque pointer to an empty struct.
  * In the API the pointer is casted to the actual C++ class.
- * \param name Name of the handle to create.
+ * The 'name' argument to the macro is the name of the handle to create.
  */
 
 GHOST_DECLARE_HANDLE(GHOST_SystemHandle);