Update send-sync.md (#258)

Adding notes that these traits are not to be implemented directly. Yet they are still to be used as normal traits in generic constraints.

Author: igor-petruk

src/concurrency/send-sync.md

@@ -7,5 +7,16 @@ How does Rust know to forbid shared access across thread? The answer is in two t
 * [`Sync`][2]: a type `T` is `Sync` if it is safe to move a `&T` across a thread
   boundary.
 
+Both traits are not to be implemented. They are implemented automatically when 
+the compiler determines it’s appropriate.
+
 [1]: https://doc.rust-lang.org/std/marker/trait.Send.html
 [2]: https://doc.rust-lang.org/std/marker/trait.Sync.html
+
+<details>
+
+* One can think of these traits as markers that the type has certain
+  thread-safety properties.
+* They can be used in the generic constraints as normal traits.
+  
+</details>