Clarify rand-extension comment wrt subject argument

examples/rand-extension/lib.rs

@@ -86,7 +86,8 @@ mod rand_extension {
             Self::new(Default::default())
         }
 
-        /// Update the value from the runtimes random source.
+        /// Seed a random value by passing some known argument `subject` to the runtime's random
+        /// source. Then, update the current value with the new random value
         #[ink(message)]
         pub fn update(&mut self, subject: [u8; 32]) -> Result<(), RandomReadErr> {
             // Get the on-chain random seed