Documenting the virtual option for types where it makes sense.

This PR includes the documentation of the `virtual` option for those types which make sense to use with this option. When this is merged upstream, part two of #1387 can be appproached by adding the mapped documentation to the 2.1 branch.

| Q             | A
| ------------- | ---
| Doc fix?      | yes (Improvementt on existing docs)
| New docs?  |   yes
| Applies to    | 2.0+
| Fixed tickets | #1387 (incomplete)