Add book section and fix typo.

Author: repnop

src/dlmalloc

@@ -417,3 +417,7 @@ JavaScript, and font files in a single location, rather than duplicating it once
 (grouping of crate docs generated into the same output directory, like with `cargo doc`). Per-crate
 files like the search index will still load from the documentation root, but anything that gets
 renamed with `--resource-suffix` will load from the given path.
+
+### `--persist-doctests`: persist doctest executables after running
+
+This feature allows the persistence of the doctest executables to the specified path.

src/doc/rustdoc/src/unstable-features.md

@@ -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/libcompiler_builtins

@@ -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",