Auto merge of #4376 - behnam:virtual, r=alexcrichton

[docs/manifest] Add Virtual Manifest section

Current documentation does not mention Virtual Manifests at all, which
can be confusing because they appear in error messages.

Here we add the definition, and provide a hint about `--all` option for
most cargo commands, which allow the command to work in the way most
probably expected.

Fixes <https://github.com/rust-lang/cargo/issues/3526>

diff --cc src/doc/manifest.md
Simple merge