alternator: complement rjson.hh comments
alternator: complement rjson.hh comments

Some comments in rjson.hh header file were not clear and are hereby
amended.
Message-Id: <5ae893e6c98b425d955180cecdf3421e682edd97@scylladb.com>

scylladb/scylla,avikivity/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla