Document using parallel commands in fab with `-P`

Fab supports parallel command execution, which is a lot faster in most of the cases:
```
fab -P pg.restart

[localhost] Executing task 'pg.restart'
[10.192.0.5] Executing task 'pg.restart'
[10.192.0.4] Executing task 'pg.restart'
[10.192.0.7] Executing task 'pg.restart'
...
```

Whereas we today instruct sequential:
```
fab pg.restart
[localhost] Executing task 'pg.restart'
[localhost] run: readlink /home/pguser/pg-latest
[localhost] out: /home/pguser/citus-installation
[localhost] out:

[localhost] run: set -m; bin/pg_ctl -D data -l logfile restart
[localhost] out: waiting for server to shut down.... done
[localhost] out: server stopped
[localhost] out: waiting for server to start.... done
[localhost] out: server started
[localhost] out:

[10.192.0.5] Executing task 'pg.restart'
[10.192.0.5] run: readlink /home/pguser/pg-latest
[10.192.0.5] out: /home/pguser/citus-installation
[10.192.0.5] out:

[10.192.0.5] run: set -m; bin/pg_ctl -D data -l logfile restart
[10.192.0.5] out: waiting for server to shut down.... done
[10.192.0.5] out: server stopped
[10.192.0.5] out: waiting for server to start.... done
[10.192.0.5] out: server started
[10.192.0.5] out:

[10.192.0.4] Executing task 'pg.restart'
[10.192.0.4] run: readlink /home/pguser/pg-latest
[10.192.0.4] out: /home/pguser/citus-installation
[10.192.0.4] out:

[10.192.0.4] run: set -m; bin/pg_ctl -D data -l logfile restart
[10.192.0.4] out: waiting for server to shut down.... done
[10.192.0.4] out: server stopped
[10.192.0.4] out: waiting for server to start.... done
[10.192.0.4] out: server started
[10.192.0.4] out:

[10.192.0.7] Executing task 'pg.restart'
[10.192.0.7] run: readlink /home/pguser/pg-latest
[10.192.0.7] out: /home/pguser/citus-installation
[10.192.0.7] out:

[10.192.0.7] run: set -m; bin/pg_ctl -D data -l logfile restart
[10.192.0.7] out: waiting for server to shut down.... done
[10.192.0.7] out: server stopped
[10.192.0.7] out: waiting for server to start.... done
[10.192.0.7] out: server started
[10.192.0.7] out:

Done.
Disconnecting from 10.192.0.7... done.
Disconnecting from 10.192.0.5... done.
Disconnecting from localhost... done.
Disconnecting from 10.192.0.4... done.
```