Auto merge of #7623 - ehuss:doc-target-json-dep, r=alexcrichton

Update documentation for custom target dependencies.

Closes #7613.

The old documentation was wrong, and hasn't worked for some time. It needs to be the file stem (see issue for discussion).

I also added a test, since I couldn't find any.  It was remarkably awkward to add the test without build-std, so I just added it to a build-std test.