Add language to MVP.md about failure modes

proposals/gc/MVP.md

@@ -493,6 +493,8 @@ In particular, `ref.null` is typed as before, despite the introduction of `none`
 
 #### Structures
 
+Note: Allocation of memory objects may fail for implementation-specific reasons, such as system resource exhaustion or a host-defined limit, or being unable to fit a large array in available address space. Similar to stack overflow, allocation failure may materialise in an implementation-dependent way, such as trapping or terminating the host process. Implementors should be aware this may be used as a denial of service vector.
+
 * `struct.new <typeidx>` allocates a structure with canonical [RTT](#values) and initialises its fields with given values
   - `struct.new $t : [t'*] -> [(ref $t)]`
     - iff `expand($t) = struct (mut t'')*`