concurrency: Add detailed teaching notes for welcome and threads slides (#1885)

These follow the flow of what I actually teach, which spends a
significant amount of time on the latter slide. I think it's worthwhile
to have a real flow documented in the teaching notes, both to make sure
nothing gets forgotten and to structure the experience of teaching.

src/concurrency.md

@@ -11,3 +11,13 @@ channels.
 The Rust type system plays an important role in making many concurrency bugs
 compile time bugs. This is often referred to as _fearless concurrency_ since you
 can rely on the compiler to ensure correctness at runtime.
+
+<details>
+
+- Rust lets us access OS concurrency toolkit: threads, sync. primitives, etc.
+- The type system gives us safety for concurrency without any special features.
+- The same tools that help with "concurrent" access in a single thread (e.g., a
+  called function that might mutate an argument or save references to it to read
+  later) save us from multi-threading issues.
+
+</details>