Add a concrete example of an optional value. Fixes #2848.

nostarch/chapter06.md

@@ -265,10 +265,16 @@ useful: `Option`.
 
 This section explores a case study of `Option`, which is another enum defined
 by the standard library. The `Option` type encodes the very common scenario in
-which a value could be something or it could be nothing. Expressing this
-concept in terms of the type system means the compiler can check whether you’ve
-handled all the cases you should be handling; this functionality can prevent
-bugs that are extremely common in other programming languages.
+which a value could be something or it could be nothing.
+<!-- Liz: I just got an issue from a reader requesting a concrete example
+of this scenario, so I've added two sentences here. Please check to see if they
+make sense! /Carol-->
+For example, if you request the first of a list containing items, you would get
+a value. If you request the first item of an empty list, you would get nothing.
+Expressing this concept in terms of the type system means the compiler can
+check whether you’ve handled all the cases you should be handling; this
+functionality can prevent bugs that are extremely common in other programming
+languages.
 
 Programming language design is often thought of in terms of which features you
 include, but the features you exclude are important too. Rust doesn’t have the

src/ch06-01-defining-an-enum.md

@@ -183,7 +183,9 @@ useful: `Option`.
 
 This section explores a case study of `Option`, which is another enum defined
 by the standard library. The `Option` type encodes the very common scenario in
-which a value could be something or it could be nothing. Expressing this
+which a value could be something or it could be nothing. For example, if you
+request the first of a list containing items, you would get a value. If you
+request the first item of an empty list, you would get nothing. Expressing this
 concept in terms of the type system means the compiler can check whether you’ve
 handled all the cases you should be handling; this functionality can prevent
 bugs that are extremely common in other programming languages.