Add current behavior for alloc_stack explicitly

Author: kitasuke

docs/SIL.rst

@@ -1886,6 +1886,8 @@ Allocates uninitialized memory that is sufficiently aligned on the stack
 to contain a value of type ``T``. The result of the instruction is the address
 of the allocated memory.
 
+``alloc_stack`` always allocates memory on the stack even for runtime-sized type.
+
 ``alloc_stack`` marks the start of the lifetime of the value; the
 allocation must be balanced with a ``dealloc_stack`` instruction to
 mark the end of its lifetime. All ``alloc_stack`` allocations must be