improve description of --initial-cluster-state flag

[lavacat]

What would you like to be added?

Currently etcd -h show following description of --initial-cluster-state flag

  --initial-cluster-state 'new'
    Initial cluster state ('new' or 'existing').

But this poorly summarizes logic of bootsrapCluster.

1. Description just repeats the name of the flag.
2. The flag is only applicable when Wal doesn't exist/readonly, but that's never mentioned.
3. Meaning of new and existing isn't clear.

Why is this needed?

I think folks are getting confused by the meaning of this flag. See #15705 (comment)

[sharathsivakumar]

I am interested to work on this. @jmhbnz

[lavacat]

keeping the issue open to track backports #15752 (comment)