qmp: document strict parsing

capabilities array (section '2.2 Server Greeting'). Thus, Clients can check

that array and enable the capabilities they support.

Additionally, Clients must not assume any particular:

- Size of json-objects or length of json-arrays

The QMP Server performs a type check on the arguments to a command.  It

generates an error if a value does not have the expected type for its

key, or if it does not understand a key that the Client included.  The

strictness of the Server catches wrong assumptions of Clients about

the Server's schema.  Clients can assume that, when such validation

errors occur, they will be reported before the command generated any

side effect.

However, Clients must not assume any particular:

- Length of json-arrays

- Size of json-objects; in particular, future versions of QEMU may add

  new keys and Clients should be able to ignore them.

- Order of json-object members or json-array elements

- Amount of errors generated by a command, that is, new errors can be added

  to any existing command in newer versions of the Server

Of course, the Server does guarantee to send valid JSON.  But apart from

this, a Client should be "conservative in what they send, and liberal in

what they accept".

6. Downstream extension of QMP

------------------------------