Document ability to use groups in 'select options' fields

I wanted to create a select list where the options were grouped into different categories. E.g.:

First group
  Option one
  Option two
Second group
  Option three
  Option four
  etc.

I found this for Drupal, so copied and pasted that into the manual entry field for my select options field and it worked. I then noticed when changing back to the normal entry that groups are created by dragging options under others (indented). But there's no mention of this ability (that I could see).

I'd like to suggest adding some help text to the select options field settings form that describes this ability.

[herbdool]

Sounds good. Would you like to take a crack at a PR?