Update unsafe.md (#252)

* Update unsafe.md

Adding a paragraph explaining that unsafe code is not necessary broken or evil, but it is a mode
where compiler safety features are off.

* Move explanation to speaker notes

To avoid slide being too long. Also edited text slightly.

* Remove extra space

---------

Co-authored-by: Andrew Walbran <qwandor@google.com>

src/unsafe.md

@@ -8,6 +8,9 @@ The Rust language has two parts:
 We will be seeing mostly safe Rust in this course, but it's important to know
 what Unsafe Rust is.
 
+Unsafe code is usually small and isolated, and its correctness should be carefully
+documented. It is usually wrapped in a safe abstraction layer.
+
 Unsafe Rust gives you access to five new capabilities:
 
 * Dereference raw pointers.
@@ -16,6 +19,14 @@ Unsafe Rust gives you access to five new capabilities:
 * Call `unsafe` functions, including `extern` functions.
 * Implement `unsafe` traits.
 
-We will briefly cover these capabilities next. For full details, please see
+We will briefly cover unsafe capabilities next. For full details, please see
 [Chapter 19.1 in the Rust Book](https://doc.rust-lang.org/book/ch19-01-unsafe-rust.html)
 and the [Rustonomicon](https://doc.rust-lang.org/nomicon/).
+
+<details>
+
+Unsafe Rust does not mean the code is incorrect. It means that developers have
+turned off the compiler safety features and have to write correct code by
+themselves. It means the compiler no longer enforces Rust's memory-safety rules.
+
+</details>