Document link to unstable book

Author: camsteffen

src/doc/rustc/src/command-line-arguments.md

@@ -302,7 +302,7 @@ _Note:_ The order of these lint level arguments is taken into account, see [lint
 This flag will allow you to set unstable options of rustc. In order to set multiple options,
 the -Z flag can be used multiple times. For example: `rustc -Z verbose -Z time-passes`.
 Specifying options with -Z is only available on nightly. To view all available options
-run: `rustc -Z help`.
+run: `rustc -Z help`, or see [The Unstable Book](../unstable-book/index.html).
 
 <a id="option-cap-lints"></a>
 ## `--cap-lints`: set the most restrictive lint level