Add documentation on the -Z flag on the command-line arguments section in the rustc book.

Author: Christian

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

@@ -86,6 +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
+
+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`.
+
 ## `--cap-lints`: set the most restrictive lint level
 
 This flag lets you 'cap' lints, for more, [see here](lints/levels.html#capping-lints).