Impact of adding a `[[test]]` section isn't documented.

[epage]

In liquid-rust, I added a [[test]] to specify the needed features to run the tests. It turns out I didn't set this up correctly, but more of issue is the fact that it disabled all of the implicit [[test]] targets without the documentation giving any hint that this will happen.

[dwijnand]

While still not documented, the impact at least triggers a runtime lint-like warning with #5330 shipped.

[ehuss]

Adding a target definition no longer disables implicit targets with the 2018 edition.

More documentation has been added for this #6898 (https://doc.rust-lang.org/nightly/cargo/reference/manifest.html#target-auto-discovery), so closing as fixed.