add plex ui access tip on rock-on write-up

[phillxnet]

When accessing the Plex Web UI via it's "Plex UI" Button on the Rock-ons page it can under some circumstances fail to show the UI and only present a Plex User Sign In / Sign Up page when there exists a functional Plex server UI. The caveat being a requirement to use the Rockstor machine's IP:
ie rather than for example with a machine named cube.lan:

http://cube.lan:32400/web/index.html

and given that machine has IP = 192.168.1.142 (from Mgmt IP: in top left of Rockstor Web UI normally)

http://192.168.1.142:32400/web/index.html

It is proposed that this 'tip' be added just below our "Please see Sign in to Your Plex Account for details."
line and possible link to this info from the Plex UI subsection. Note that similar advise is given and highlighted on the page linked to within that line:

https://support.plex.tv/hc/en-us/articles/200878643-Sign-in-to-Your-Plex-Account

"Tip!: If you aren't yet signed into your Server, you'll usually need to first access it locally via http://localhost:32400/web or http://127.0.0.1:32400/web on the Server's computer."

Although in that case the host identifiers are relevant to a local system install.

[phillxnet]

Please also update the following forum thread with this issues resolution:
https://forum.rockstor.com/t/plex-setup-issues/1777/6

[phillxnet]

A related enhancement to this doc issue would be to detail the procedure necessary to use the installed Plex server without first acquiring an online plex account:

I.e when accessing the plex server for the first time and using the above suggested 'by ip' method rather than the default via Plex UI button by hostname we are presented with the following screen:

where the indicated 'What's this" link in turn leads to:

Where upon the "Skip and accept limited functionality" linked text leads in turn to the regular and updated (since our Rock-on write-up) first intro page:

which in turn leads to the second intro / plex pass promo screen:

which when 'crossed out' (top right) gets us to the first actual plex server setup screen:

which is a slight update from our current write-up ie:

"Allow me to access my media outside my home

Plex will attempt to automatically configure your network to allow access to your Plex Media Server from Plex apps outside your home."