Document leader-election configuration

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

config/config-leader-election.yaml

@@ -21,7 +21,32 @@ metadata:
     app.kubernetes.io/instance: default
     app.kubernetes.io/part-of: tekton-pipelines
 data:
-  # An inactive but valid configuration follows; see example.
-  lease-duration: "60s"
-  renew-deadline: "40s"
-  retry-period: "10s"
+  _example: |
+    ################################
+    #                              #
+    #    EXAMPLE CONFIGURATION     #
+    #                              #
+    ################################
+    # This block is not actually functional configuration,
+    # but serves to illustrate the available configuration
+    # options and document them in a way that is accessible
+    # to users that `kubectl edit` this config map.
+    #
+    # These sample configuration options may be copied out of
+    # this example block and unindented to be in the data block
+    # to actually change the configuration.
+    # lease-duration is how long non-leaders will wait to try to acquire the
+    # lock; 15 seconds is the value used by core kubernetes controllers.
+    lease-duration: "60s"
+    # renew-deadline is how long a leader will try to renew the lease before
+    # giving up; 10 seconds is the value used by core kubernetes controllers.
+    renew-deadline: "40s"
+    # retry-period is how long the leader election client waits between tries of
+    # actions; 2 seconds is the value used by core kubernetes controllers.
+    retry-period: "10s"
+    # buckets is the number of buckets used to partition key space of each
+    # Reconciler. If this number is M and the replica number of the controller
+    # is N, the N replicas will compete for the M buckets. The owner of a
+    # bucket will take care of the reconciling for the keys partitioned into
+    # that bucket.
+    buckets: "1"