Document backoff options

[dedemorton]

Adds backup options for LS and ES outputs (closes #5419)

NOTE: I wasn't sure quite where to add the options in the yaml file.

Do we want to say anything about the backoff factor used here? Probably not since it's not configurable?

[dedemorton]

@urso I hope I got the details right this time. Right now we're ordering the options by how commonly they are used (with related options appearing next to each other). Not sure that's the best approach, but for now, I've added the options to an arbitrary location because I'm not sure how common it is for users to turn these knobs.

[urso]

Some more clarification:

backoff.init is applied on an network error. If connection was active, but connection gets lost or some other protocol/network error occurs, beats will disconnect and use backoff.init. After backoff.init, beats try to reconnect. If the reconnect fails, the backoff is increased exponentially up to backoff.max. After successful reconnect, the backoff timer is reset. On next network error, backoff will start with again with backoff.init.

A (initial) connection error is just one potential error among others.

[dedemorton]

@urso I'm trying to get all these details into the paragraph without making it too long. Does this work?

The number of seconds to wait before trying to reconnect to Elasticsearch after
a network error. After waiting `backoff.init` seconds, {beatname_uc} tries to
reconnect. If the attempt fails, the backoff timer is increased exponentially up
to `backoff.max`. After a successful connection, the backoff timer is reset. The
default is 1s.

I've left the following sentence out because it restates the previous sentence: "On next network error, backoff will start with again with backoff.init."

[urso]

@dedemorton SGTM

[dedemorton]

@urso This PR is ready for a final review.

[urso]

LGTM!

[urso]

@dedemorton The PR breaks the build. make update did update the fields.yml in winlogbeat. This file doesn't seem to be correct. Can you remove the changes to fields.yml from your PR?

[dedemorton]

@urso I had to rebase and then run make update again. I guess that's a good general best practice...that you should rebase a stale branch before running make update to pick up doc changes (even if GitHub isn't asking you to rebase).