QVarLengthArray: add missing default-ctor documentation

Was lost when we un-explicit'ed the default ctor in
c34242c679.

Pick-to: 6.2 6.1 5.15
Change-Id: Ifb4943b9e9647ae59c1cc6d5fc5076e8620b73ce
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qvarlengtharray.qdoc

@@ -90,6 +90,11 @@
     \sa QList
 */
 
+/*! \fn template<class T, qsizetype Prealloc> QVarLengthArray<T, Prealloc>::QVarLengthArray()
+
+    Constructs an array with an initial size of zero.
+*/
+
 /*! \fn template<class T, qsizetype Prealloc> QVarLengthArray<T, Prealloc>::QVarLengthArray(qsizetype size)
 
     Constructs an array with an initial size of \a size elements.