Update validator statuses info into the doc

[cuongdcdev]

As a validator, I would like to have a section in the document to explain all validator statuses, such as:

• Rollover: This means the node is currently validating and will continue validating in the next epoch.
• Proposal
• Rejected
• Rewarded
• ...
  

[DDeAlmeida]

Will add this in the new guide, thanks for the suggestion.

[walterthesmart]

@cuongdcdev Hope you are having a good day. I would like to take on this issue.