Document the default size of new wxMemoryBuffer correctly.

Documenting it as DefBufSize wasn't really helpful, so just use its value
instead (an alternative would be to document DefBufSize -- with its value --
but it doesn't seem very useful to make it part of the public API).

Closes #13984.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70603 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/buffer.h

@@ -287,9 +287,9 @@ public:
         Create a new buffer.
 
         @param size
-            size of the new buffer.
+            size of the new buffer, 1KiB by default.
     */
-    wxMemoryBuffer(size_t size = DefBufSize);
+    wxMemoryBuffer(size_t size = 1024);
 
     /**
         Append a single byte to the buffer.