Docs: remove admonition about server-members command

[brianshumate]

The warning about nomad server-members resulting in an error when run on client nodes in the Command: server-members documentation does not appear to be true any longer, so I am proposing that it be removed in this PR.

Output from a Nomad client on version 0.5.1:

nomad agent-info
client
  heartbeat_ttl = 10.450798781s
  known_servers = 10.1.42.101:4647,10.1.42.102:4647
  last_heartbeat = 7.152186302s
  node_id = af4fa74e-2382-d95c-28d5-2fa663f698c5
  num_allocations = 0
runtime
  arch = amd64
  cpu_count = 2
  goroutines = 36
  kernel.name = linux
  max_procs = 2
  version = go1.7.3

and then attempting the command on the same client node works fine:

nomad server-members
Name         Address      Port  Status  Leader  Protocol  Build  Datacenter  Region
hfb1.global  10.1.42.101  4648  alive   true    2         0.5.1  dc1         global
hfb2.global  10.1.42.102  4648  alive   false   2         0.5.1  dc1         global

[dadgar]

Thanks!

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