Add speaker notes to some concurrency pages (#2501)

Part of #1083.

src/concurrency/channels/bounded.md

@@ -35,8 +35,8 @@ fn main() {
 - Calling `send()` will block the current thread until there is space in the
   channel for the new message. The thread can be blocked indefinitely if there
   is nobody who reads from the channel.
-- A call to `send()` will abort with an error (that is why it returns `Result`)
+- Like unbounded channels, a call to `send()` will abort with an error if the
-  if the channel is closed. A channel is closed when the receiver is dropped.
+  channel is closed.
 - A bounded channel with a size of zero is called a "rendezvous channel". Every
   send will block the current thread until another thread calls [`recv()`].
 

src/concurrency/channels/unbounded.md

@@ -31,3 +31,12 @@ fn main() {
 ```
 
 [`mpsc::channel()`]: https://doc.rust-lang.org/std/sync/mpsc/fn.channel.html
+
+<details>
+
+- An unbounded channel will allocate as much space as is necessary to store
+  pending messages. The `send()` method will not block the calling thread.
+- A call to `send()` will abort with an error (that is why it returns `Result`)
+  if the channel is closed. A channel is closed when the receiver is dropped.
+
+</details>

src/concurrency/sync-exercises/dining-philosophers.md

@@ -52,3 +52,12 @@ name = "dining-philosophers"
 version = "0.1.0"
 edition = "2021"
 ```
+
+<details>
+
+- Encourage students to focus first on implementing a solution that "mostly"
+  works.
+- The deadlock in the simplest solution is a general concurrency problem and
+  highlights that Rust does not automatically prevent this sort of bug.
+
+</details>

src/concurrency/sync-exercises/link-checker.md

@@ -80,6 +80,15 @@ cargo run
   `www.google.org` domain. Put an upper limit of 100 pages or so so that you
   don't end up being blocked by the site.
 
+<details>
+
+- This is a complex exercise and intended to give students an opportunity to
+  work on a larger project than others. A success condition for this exercise is
+  to get stuck on some "real" issue and work through it with the support of
+  other students or the instructor.
+
+</details>
+
 [1]: https://docs.rs/reqwest/
 [2]: https://docs.rs/scraper/
 [3]: https://docs.rs/thiserror/