Document where `use` directives are allowed

[brson]

I expect this to work:

impl CssSelectResultsRef {                                                                                                                                                    
    use properties::CssColorProp;                                                                                                                                             
    fn computed_color(element: css_pseudo_element) -> CssColorProp {                                                                                                          
        fail                                                                                                                                                                  
    }                                                                                                                                                                         
}         

[catamorphism]

I think this is obsolete what with the new rules about where use directives are allowed. The documentation doesn't seem to be up to date. I'm going to change the bug title and nominate this for milestone 1, well-defined.

[graydon]

accepted for backwards-compatible milestone

[alexcrichton]

From reading the manual, this is well documented for use in mods, but there's no documentation indicating that these can be placed at the beginning of blocks. I also wasn't able to find a good section documenting what a block is, so those two should probably happen at the same time.