playbook: permit the serial attribute to be a pecentage string as well as well as a straight integer
Correct placement of right parenthesis.

(serial_pct)/100.0 is a typo - the parentheses should be around the entire percentage calculation, which makes much more sense when multiplying by the number of hosts (luckily BODMAS prevailed).
Add example of rolling update using a percentage.
Revise documentation versioning.