Tweak test dep docs

[IanButterworth]

Alternative proposal to #3533
Closes #3535

[IanButterworth]

It'd be good to get a 2nd opinion from a Pkg dev here, particularly marking the test/Project.toml as an "alternative", which is a bit of a demotion based on discussions like #1788

[gdalle]

I think it's fair but I understand the hesitation

[lgoettgens]

Should this change imply a deprecation of the test/Project.toml approach? If yes, I would like to see a PSA about this, and maybe some motivation behind this change. I would like to understand, why the reasons that this approach was introduced in julia 1.2 are no longer valid to keep it around.
Furthermore, I wish for a documented way of integrating other sub-projects, most notably docs/Project.toml into the main Project.toml file.

[IanButterworth]

Both approaches are valid. Just that the docs used to be worded in a way that made it seem like test/Project.toml was the new better way.

It's better to characterize it as an experimental way with caveats, caveats that were originally documented.

There's nothing wrong with using either, just that the docs now make it clearer that test/Project.toml has caveats and isn't a replacement.