update comment about using aligned_alloc

Author: RalfJung

library/std/src/sys/pal/unix/alloc.rs

@@ -92,7 +92,10 @@ cfg_if::cfg_if! {
     } else if #[cfg(target_os = "wasi")] {
         #[inline]
         unsafe fn aligned_malloc(layout: &Layout) -> *mut u8 {
-            // C11 aligned_alloc requires that the size be a multiple of the alignment.
+            // C18 allows the implementation to return null if the alignment is "not supported", and
+            // some implementations, like wasi, choose to not support the case where the size is not
+            // a multiple of the alignment. This potentially wastes a bunch of space, but there's
+            // not a lot we can do about it.
             // Layout already checks that the size rounded up doesn't overflow isize::MAX.
             let align = layout.align();
             let size = layout.size().next_multiple_of(align);