doc/jsonrpc: clarify the required, optional and error messages

SPDK implements a [JSON-RPC 2.0](http://www.jsonrpc.org/specification) server

to allow external management tools to dynamically configure SPDK components.

## Parameters

Most of the commands can take parameters. If present, parameter is validated against its domain. If this check fail

whole command will fail with response error message [Invalid params](@ref jsonrpc_error_message).

### Required parameters

These parameters are mandatory. If any required parameter is missing RPC command will fail with proper error response.

### Optional parameters

Those parameters might be omitted. If an optional parameter is present it must be valid otherwise command will fail

proper error response.

## Error response message {#jsonrpc_error_message}

Each error response will contain proper message. As much as possible the message should indicate what went wrong during

command processing.

There is ongoing effort to customize this messages but some RPC methods just return "Invalid parameters" as message body

for any kind of error.

Code   | Description

------ | -----------

-1     | Invalid state - given method exists but it is not callable in [current runtime state](@ref rpc_start_subsystem_init)

-32600 | Invalid request - not compliant with JSON-RPC 2.0 Specification

-32601 | Method not found

-32602 | @ref jsonrpc_invalid_params

-32603 | Internal error for e.g.: errors like out of memory

-32700 | @ref jsonrpc_parser_error

### Parser error {#jsonrpc_parser_error}

Encountered some error during parsing request like:

- the JSON object is malformed

- parameter is too long

- request is too long

### Invalid params {#jsonrpc_invalid_params}

This type of error is most common one. It mean that there is an error while processing the request like:

- Parameters decoding in RPC method handler failed because required parameter is missing.

- Unknown parameter present encountered.

- Parameter type doesn't match expected type e.g.: given number when expected a string.

- Parameter domain check failed.

- Request is valid but some other error occurred during processing request. If possible message explains the error reason nature.

# App Framework {#jsonrpc_components_app}

## kill_instance {#rpc_kill_instance}