Write end user Web-UI embeded system shell doc

[phillxnet]

Our current doc entry for the build-in shellinabox currently reads: "Coming soon…". Not exactly that helpful.

It is proposed that we have a short run down on what this is and how to configure it. I.e. we have the two major personalities for this where the shell either runs as a normal user or, in non default mode, as the root user. We could include a note for folks to "su" "su -" [EDIT @phillxnet ] to the "root" user so they can access for example the btrfs command.

[FroggyFlox]

@phillxnet , I'm linking to your excellent detailing on how to do just that on the forum:
https://forum.rockstor.com/t/poetry-cant-delete-rock-on/8719/7

[phillxnet]

@FroggyFlox I've done a little edit here to ensure folks invoke root's environment (for the most part) as per the failing we saw in the forum thread you linked to. Folks mostly will be expecting this, and where they know otherwise they can see our use of the --login short hand. Unless of course we use it's full form. But that will be in the hands of the do-ocracy dooer.