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.

web/news.txt

@@ -20,7 +20,11 @@ Changes affecting backwards compatibility
   new experimental ``this`` pragma to achieve a similar effect to what the old ``using`` statement tried to achieve.
 - Typeless parameters have been removed from the language since it would
   clash with ``using``.
-
+- When using ``useMalloc``, an additional header containing the size of the
+  allocation will be allocated, to support zeroing memory on realloc as expected
+  by the language. With this change, ``alloc`` and ``dealloc`` are no longer
+  aliases for ``malloc`` and ``free`` - use ``c_malloc`` and ``c_free`` if
+  you need that.
 
 Library Additions
 -----------------