Document or enable commented-out sanity check in checkStackChunk

checkStackChunk is currently defined like this:

void
checkStackChunk( StgPtr sp, StgPtr stack_end )
{
    StgPtr p;

    p = sp;
    while (p < stack_end) {
        p += checkStackFrame( p );
    }
    // ASSERT( p == stack_end ); -- HWL
}

The commented-out assertion makes perfect sense to me, but I can't enable it (causes test failures), and it's not documented why it should not be enabled. I think this catches a bug that needs to be fixed.