Minor fixes in new lifetimes section (#2965)

@gribozavr I had a couple more fixes for
https://github.com/google/comprehensive-rust/pull/2964 that I didn't
push until after you merged the branch.

src/lifetimes/borrow-both.md

@@ -8,7 +8,7 @@ In this case, we have a function where either `a` or `b` may be returned. In
 this case we use the lifetime annotations to tell the compiler that both borrows
 may flow into the return value.
 
-```rust
+```rust,editable
 fn pick<'a>(c: bool, a: &'a i32, b: &'a i32) -> &'a i32 {
     if c { a } else { b }
 }

src/lifetimes/borrow-one.md

@@ -68,10 +68,16 @@ fn main() {
   enforcing that the function adheres to the contract set by the function's
   signature.
 
-- The "help" note in the error notes that we can add a lifetime bound `'b: 'a`
-  to say that `'b` will live at least as long as `'a`, which would then allow us
-  to return `query`. On the next slide we'll talk about lifetime variance, which
-  is the rule that allows us to return a longer lifetime when a shorter one is
-  expected.
+# More to Explore
+
+- The "help" message in the error notes that we can add a lifetime bound
+  `'b: 'a` to say that `'b` will live at least as long as `'a`, which would then
+  allow us to return `query`. This is an example of lifetime subtyping, which
+  allows us to return a longer lifetime where a shorter one is expected.
+
+- We can do something similar by returning a `'static` lifetime, e.g., a
+  reference to a `static` variable. The `'static` lifetime is guaranteed to be
+  longer than any other lifetime, so it's always safe to return in place of a
+  shorter lifetime.
 
 </details>

src/lifetimes/lifetime-elision.md

@@ -2,7 +2,7 @@
 minutes: 5
 ---
 
-# Lifetimes in Function Calls
+# Lifetime Elision
 
 Lifetimes for function arguments and return values must be fully specified, but
 Rust allows lifetimes to be elided in most cases with

src/lifetimes/multiple-borrows.md

@@ -7,7 +7,7 @@ minutes: 5
 But what about when there are multiple borrows passed into a function and one
 being returned?
 
-```rust,editable,ignore
+```rust,editable,compile_fail
 fn multiple(a: &i32, b: &i32) -> &i32 {
     todo!("Return either `a` or `b`")
 }