Add assert to make doc test fails.

src/parse/matches/arg_matches.rs

@@ -710,6 +710,7 @@ impl ArgMatches {
     ///             // clap sees the above as 'myapp -o val1 val2 val3'
     ///             //                         ^0    ^1 ^2   ^3   ^4
     /// assert_eq!(m.index_of("option"), Some(2));
+    /// assert_eq!(m.indices_of("option").unwrap().collect::<Vec<_>>(), &[2, 3, 4]);
     /// ```
     /// [`ArgMatches`]: ./struct.ArgMatches.html
     /// [delimiter]: ./struct.Arg.html#method.value_delimiter