docs: Clarify some expections of the json_object_get_*_member APIs
It is an error to use the following with a missing member:
• json_object_get_int_member • json_object_get_double_member • json_object_get_boolean_member • json_object_get_null_member • json_object_get_string_member • json_object_get_array_member • json_object_get_object_member
Doing so will lead to CRITICALs like these: Json-CRITICAL **: json_object_get_string_member: assertion 'node != NULL' failed
Instead one of these should be used to determine the existence of the member:
• json_object_get_member • json_object_has_member
Fixes #24 (closed)