Updated the description of -Z in the rustc book.

Author: Christian

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

@@ -86,11 +86,12 @@ This flag will set which lints should be set to the [deny level](lints/levels.ht
 
 This flag will set which lints should be set to the [forbid level](lints/levels.html#forbid).
 
-## `-Z`: set options for debugging rustc
+## `-Z`: set unstable options
 
-This flag lets you set options for debugging rustc. In order to set multiple options, the -Z flag
-can be used multiple times. For example: `rustc -Z verbose -Z time`. Specifying options with -Z is
-only available on nightly. To view all available options run: `rustc -Z help`.
+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`.
+Specifying options with -Z is only available on nightly. To view all available options
+run: `rustc -Z help`.
 
 ## `--cap-lints`: set the most restrictive lint level