Follow-up from "Edited for grammar and style"

The following discussions from !2830 (merged) should be addressed:

The `session_server` section is not supported by the

@marcel.amirault started a discussion:

It seemed to make sense to have the example after the table?

@marcel.amirault started a discussion:

WDYT about moving your new column into the middle? I think it renders poorly the way you have it here:

If we move it into the middle:

So how about this, along with:

change Requires to Required configuration, for clarity?
change Where the build runs to Where builds run?

| Executor             | Required configuration                                                                     | Where builds run |
|----------------------|--------------------------------------------------------------------------------------------|----------------------|
| `shell`              |                                                                                            | Local shell. The default executor. |
| `docker`             | `[runners.docker]` and [Docker Engine](https://docs.docker.com/engine/)                    | A Docker container. |
| `docker-windows`     | `[runners.docker]` and [Docker Engine](https://docs.docker.com/engine/)                    | A Windows Docker container. |
| `docker-ssh`         | `[runners.docker]`, `[runners.ssh]`, and  [Docker Engine](https://docs.docker.com/engine/) | A Docker container, but connect with SSH. <br>**The Docker container runs on the local machine. This setting changes how the commands are run inside that container. If you want to run Docker commands on an external machine, change the  `host`  parameter in the  `runners.docker`  section.** |
| `ssh`                | `[runners.ssh]`                                                                            | SSH, remotely. |
| `parallels`          | `[runners.parallels]` and `[runners.ssh]`                                                  | Parallels VM, but connect with SSH. |
| `virtualbox`         | `[runners.virtualbox]` and `[runners.ssh]`                                                 | VirtualBox VM, but connect with SSH. |
| `docker+machine`     | `[runners.docker]` and `[runners.machine]`                                                 | Like `docker`, but use [auto-scaled Docker machines](autoscale.md). |
| `docker-ssh+machine` | `[runners.docker]` and `[runners.machine]`                                                 | Like `docker-ssh`, but use [auto-scaled Docker machines](autoscale.md). |
| `kubernetes`         | `[runners.kubernetes]`                                                                     | Kubernetes pods. |