Document leader-election configuration

[dibyom]

Changes

The leader-election config did not describe what the options did and were set to the default values. See https://github.com/knative/pkg/blob/e60d250dc6378c387c8c9d149774ee21e9a827ab/leaderelection/config.go#L121-L127
This commit adds block of commented out example config (like we do in config-defaults). The actual content was taken from https://github.com/knative/serving/blob/f9951ecf37709bb160fd513591e84b6e6f50f6f8/config/core/configmaps/leader-election.yaml

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As the author of this PR, please check off the items in this checklist:

• Has Docs included if any changes are user facing
• Has Tests included if any functionality added or changed
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Release Notes

NONE

[dibyom]

/kind misc

[lbernick]

Are we currently actively using these configuration options, and this PR is replacing them with invalid configuration? Is this documentation code block intended for tekton developers or cluster operators using tekton?

[tekton-robot]

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[dibyom]

Are we currently actively using these configuration options,

Yes, we support a HA mode. The config file was essentially a no-op since it was supplying default values - those values are also present in the code.

and this PR is replacing them with invalid configuration? Is this documentation code block intended for tekton developers or cluster operators using tekton?

This is a configuration with no values specified which means the HA code will use the default config from the code. The docs are mostly intended towards cluster operators.

[abayer]

/lgtm