Add documentation for disable_retry_limit option.

- Documented the usage of the new disable_retry_limit option
  for output plugins.
- Refactored the documentation on retry_wait/max_retry_wait and
  retry_limit/disable_retry_limit a bit since it was awkward
  with 4 options in one section.
- Documented the +/- 12.5% randomness in the retry interval.