Improve RAPI documentation on users and options

- Document fields in “rapi/users” file in the order they appear
  (username, password, options)
- Mention new “--require-authentication” option
- Clarify use of “write” on “GET” method

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>