Samples in docs/samples.users.md do not work

[jasonjoh]

• The command given is mg, but should be mgc.
• The required scope of users.readwrite.all (to execute all 3) isn't mentioned.
• The update sample requires setting two session variables ($userId and $newLocation). This isn't mentioned.
• The first sample mgc users user list --query 'length([])' gives an error: mgc users user list: error: argument --query: invalid jmespath_type value: 'length([]'.
• The third sample mgc users user update --user-id $userId --office-location $newLocation gives an error: (BadRequest) The request is currently not supported on the targeted entity set.
  AB#10534