Improve RAPI documentation for command line usage

Provide some more insights about how to use the RAPI from the command line
through the "curl" command.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>

doc/rapi.rst

@@ -75,7 +75,8 @@ When using the RAPI, username and password can be sent to the server
 by using the standard HTTP basic access authentication. This means that
 for accessing the protected URL ``https://cluster.example.com/resource``,
 the address ``https://username:password@cluster.example.com/resource`` should
-be used instead.
+be used instead. Alternatively, the appropriate parameter of your HTTP client
+(such as ``-u`` for ``curl``) can be used.
 
 .. [#pwhash] Using the MD5 hash of username, realm and password is
    described in :rfc:`2617` ("HTTP Authentication"), sections 3.2.2.2
@@ -279,7 +280,10 @@ or ``curl``::
   $ curl https://%CLUSTERNAME%:5080/2/info
 
 Note: with ``curl``, the request method (GET, POST, PUT) can be specified
-using the ``-X`` command line option.
+using the ``-X`` command line option, and the username/password can be
+specified with the ``-u`` option. In case of POST requests with a body, the
+Content-Type can be set to JSON (as per the Protocol_ section) using the
+parameter ``-H "Content-Type: application/json"``.
 
 Python
 ++++++