[DOCS] Added important updateStateByKey details

Runs for *all* existing keys and returning "None" will remove the key-value pair.

Author: Michael Vogiatzis <michaelvogiatzis@gmail.com>

Closes #7229 from mvogiatzis/patch-1 and squashes the following commits:

e7a2946 [Michael Vogiatzis] Updated updateStateByKey text
00283ed [Michael Vogiatzis] Removed space
c2656f9 [Michael Vogiatzis] Moved description farther up
0a42551 [Michael Vogiatzis] Added important updateStateByKey details