Update macro tutorial (+ minor release note edit)#4123

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paulstansifer wants to merge 3 commits intorust-lang:incomingfrom

paulstansifer:update_macro_tutorial

Commits on Dec 6, 2012

Update macro tutorial to recognize item/statement macros and be clearer
paulstansifer
committed
Mention statement macros also in release notes, for completeness.
paulstansifer
committed
Be a bit more explicit about the nature of macro RHSes.
paulstansifer
committed