doc(vhpidirect/mistakes): add info about size limits

doc/vhpidirect/mistakes.rst

@@ -13,3 +13,12 @@ Common mistakes
   subprogram **must** be defined (linked) for elaboration to succeed. This is required even when it is not used in the
   design. In context where conditional inclusion of VHPIDIRECT features is required, it is suggested to provide
   alternative (dummy) sources for either VHDL packages or C sources (see :ghdlsharp:`793`).
+
+* There are some corner cases when using large variables/signals. When you try to allocate a single object which is too
+  large, GHDL will complain. That can be fixed with :option:`--max-stack-alloc`. However, when you allocate multiple
+  objects which are smaller than the limit, but the last one overflows, GHDL will not complain, it will crash instead.
+  In this case, ``ulimit`` needs to be modified/increased. To remove the limit, set ``ulimit -s unlimited``.
+  See :ghdlsharp:`1112`.
+
+* The size of objects is supposed to be 4GB. However, users finding the limit might get different constraints, depending
+  on the backend. See :ghdlsharp:`822`.