Update Force Leave docs (#6550)

Fixes #2742

Previously the docs didn't clarify that if a server restarts as a client then force-leave won't lead to removing the node from the raft config. This is because the node, which is alive after a restart, will refute messages about it having left . These messages about members leaving are in turn what trigger Consul's leader to remove a server from raft.

website/source/docs/commands/force-leave.html.markdown.erb

@@ -11,19 +11,23 @@ description: |-
 Command: `consul force-leave`
 
 The `force-leave` command forces a member of a Consul cluster to enter the
-"left" state. If the member is still actually alive, it will
+"left" state. The purpose of this method is to force-remove a node that has failed or 
-eventually rejoin the cluster. The true purpose of this method is to force
+was shutdown without a [graceful leave](https://www.consul.io/docs/commands/leave.html).
-remove "failed" nodes.
 
-Consul periodically tries to reconnect to "failed" nodes in case it is a
+Consul periodically tries to reconnect to "failed" nodes in case failure was due
-network partition. After some configured amount of time (by default 72 hours),
+to a network partition. After some configured amount of time (by default 72 hours),
 Consul will reap "failed" nodes and stop trying to reconnect. The `force-leave`
-command can be used to transition the "failed" nodes to "left" nodes more
+command can be used to transition the "failed" nodes to a "left" state more
-quickly.
+quickly, as reported by [`consul memebers`](https://www.consul.io/docs/commands/members.html).
 
 This can be particularly useful for a node that was running as a server,
-as it will be removed from the Raft quorum. Note that `force-leave` cannot be
+as it will eventually be removed from the Raft configuration by the leader.
-used to force removal of nodes that are outside of the datacenter.
+
+~> Note that for `force-leave` to take full effect the target node's agent must have
+shutdown permanently. If the agent is alive and reachable then it will not be removed
+from the datacenter's member list nor from the raft configuration. Additionally,
+if the agent returns after transitioning to the "left" state, but before it is reaped
+from the member list, then it will rejoin the cluster.
 
 ## Usage
 
@@ -33,3 +37,22 @@ Usage: `consul force-leave [options] node`
 
 <%= partial "docs/commands/http_api_options_client" %>
 
+## Examples
+
+Remove a node named `ec2-001-staging` from the local agent's datacenter:
+
+```
+consul force-leave ec2-001-staging
+```
+
+When run on a server that is part of a
+[WAN gossip pool](https://learn.hashicorp.com/consul/security-networking/datacenters),
+`force-leave` can remove failed servers in other datacenters from the WAN pool.
+
+The identifying node-name in a WAN pool is `[node-name].[datacenter]`.
+Therefore, to remove a failed server node named `server1` from
+datacenter `us-east1`, run: 
+
+```
+consul force-leave server1.us-east1
+```