document useMalloc changes

doc/nimc.txt

@@ -243,7 +243,9 @@ Define               Effect
 ``useFork``          Makes ``osproc`` use ``fork`` instead of ``posix_spawn``.
 ``useNimRtl``        Compile and link against ``nimrtl.dll``.
 ``useMalloc``        Makes Nim use C's `malloc`:idx: instead of Nim's
-                     own memory manager. This only works with ``gc:none``.
+                     own memory manager, ableit prefixing each allocation with
+                     its size to support clearing memory on reallocation.
+                     This only works with ``gc:none``.
 ``useRealtimeGC``    Enables support of Nim's GC for *soft* realtime
                      systems. See the documentation of the `gc <gc.html>`_
                      for further information.