json-writer: describe the usage of jw_* functions

json-writer: describe the usage of jw_* functions

Provide an overview of the set of functions used for manipulating
`json_writer`s, by describing what functions should be used for
each JSON-related task.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Patrick Steinhardt <ps@pks.im>
Helped-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Lucas Seiki Oshiro <lucasseikioshiro@gmail.com>
Acked-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>