Rustdoc should emit macro documentation

[orenbenkiki]

It is now possible to add attributes to macros, so it is possible to add documentation to each one. The rustdoc tool should emit the macro documentation in a "MACROS" section (probably just before the "FUNCTIONS" section).

[alexcrichton]

This is a little tricky because all macros are lost after crate expansion. This would probably involve leaving behind sentinel values which are ignored for the rest of compilation, but remain to have the documentation.

There's also the matter of what the documentation should show, and it should probably show the patterns for which the macro is defined (not just the name of the macro), and this is a bit tricky as well. Just things to consider though, don't mean to stop any progress on this!

It would be awesome to have std::macros define all the macros defined by the compiler so they could actually all be documented in one place!

[huonw]

We now have exportable macros, so being able to document them is important.

[huonw]

Also, cc #3163.

[sfackler]

Exported macro definitions are now left in the AST after expansion, so this should be easier.