Doctests aren't running for documentation of macros

[sgrif]

I noticed that in a branch on Diesel where I had forgotten to update a few doctests, our builds were passing on nightly but failing on stable and beta. It appears that the doctests on macros are not being run on the current nightly. https://travis-ci.org/diesel-rs/diesel/builds/181708974

[alexcrichton]

cc @jseyfried, does this ring a bell for you?

[jseyfried]

This was caused by #37890, fixed in #38239.

[alexcrichton]

Thanks for the quick fix @jseyfried!