Add comment for the const

Author: tiif

src/shims/unix/linux/eventfd.rs

@@ -10,6 +10,7 @@ use crate::{concurrency::VClock, *};
 
 use self::shims::unix::fd::FileDescriptor;
 
+// Size of u8 array to hold u64.
 const U64_ARRAY_SIZE: usize = mem::size_of::<u64>();
 
 /// Maximum value that the eventfd counter can hold.