Add book section and fix typo.

src/librustdoc/config.rs

@@ -68,7 +68,8 @@ pub struct Options {
     pub should_test: bool,
     /// List of arguments to pass to the test harness, if running tests.
     pub test_args: Vec<String>,
-    /// Otional path to persist the doctest executables to.
+    /// Optional path to persist the doctest executables to, defaults to a
+    /// temporary directory if not set.
     pub persist_doctests: Option<PathBuf>,
 
     // Options that affect the documentation process

src/librustdoc/lib.rs

@@ -340,6 +340,7 @@ fn opts() -> Vec<RustcOptGroup> {
             o.optflag("",
                       "disable-per-crate-search",
                       "disables generating the crate selector on the search box")
+        }),
         unstable("persist-doctests", |o| {
              o.optopt("",
                        "persist-doctests",