Update rustdoc book content about --extend-css option

Author: GuillaumeGomez

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

@@ -320,10 +320,7 @@ $ rustdoc src/lib.rs --extend-css extra.css
 ```
 
 With this flag, the contents of the files you pass are included at the bottom
-of Rustdoc's `theme.css` file.
-
-While this flag is stable, the contents of `theme.css` are not, so be careful!
-Updates may break your theme extensions.
+of the `theme.css` file.
 
 ## `--sysroot`: override the system root