Add macro scope details to the Macro tutorial.

[nejucomo]

The Macro Tutorial doesn't explain the scope of macro definitions, nor does it describe the #[macro_escape] mechanism.

[pnkfelix]

part of #11755

(though i'm not 100% sure that the macro_escape mechanism is properly within the scope of a tutorial, rather than a reference manual.... also I'm not sure if the macro_escape mechanism is something we want to commit to supporting long term...)

[kmcallister]

also I'm not sure if the macro_escape mechanism is something we want to commit to supporting long term

I'd recommend a rename at minimum. macro_escape vs macro_export is very confusing.

[steveklabnik]

http://doc.rust-lang.org/book/macros.html#scoping-and-macro-import/export now describes scope, as well as macro_use, the fixed up version of macro-escape.