auto merge of #13255 : alexcrichton/rust/issue-5605, r=huonw

These syntax extensions need a place to be documented, and this starts passing a
`--cfg dox` parameter to `rustdoc` when building and testing documentation in
order to document macros so that they have no effect on the compiled crate, but
only documentation.

Closes #5605

diff --cc src/librustdoc/lib.rs
Simple merge