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Document qubes.StartApp security advantages

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Thanks to @marmarek and @jpouellet for providing these explanations:

https://groups.google.com/d/topic/qubes-devel/BsrjCN3Vg_g/discussion
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@ -167,6 +167,35 @@ be fatal to Qubes security. On the other hand, this mechanism allows to
delegate processing of untrusted input to less privileged (or disposable)
AppVMs, thus wise usage of it increases security.
For example, this command will run the `firefox` command in a DisposableVM based
on `work`:
```
$ qvm-run --dispvm=work firefox
```
By contrast, consider this command:
```
$ qvm-run --dispvm=work --service qubes.StartApp+firefox
```
This will look for a `firefox.desktop` file in a standard location in a
DisposableVM based on `work`, then launch the application described by that
file. The practical difference is that the bare `qvm-run` command uses the
`qubes.VMShell` service, which allows you to run an arbitrary command with
arbitrary arguments, essentially providing full control over the target VM. By
contrast, the `qubes.StartApp` service allows you to run only applications that
are advertised in `/usr/share/applications` (or other standard locations)
*without* control over the arguments, so giving a VM access to `qubes.StartApp`
is much safer. While there isn't much practical difference between the two
commands above when starting an application from dom0 in Qubes 4.0, there is a
significant security risk when launching applications from a domU (e.g., from
a separate GUI domain). This is why `qubes.StartApp` uses our standard `qrexec`
argument grammar to strictly filter the permissible grammar of the `Exec=` lines
in `.desktop` files that are passed from untrusted domUs to dom0, thereby
protecting dom0 from command injection by maliciously-crafted `.desktop` files.
### Extra keywords available in Qubes 4.0 and later
**This section is about a not-yet-released version, some details may change**