reader: Clarify use of json_reader_end_[element|member]() with errors
reader: Clarify use of json_reader_end_[element|member]() with errors

Include an example showing that the end function must be called on both
code paths (success and failure) if the reader object is to be used
afterwards.

https://bugzilla.gnome.org/show_bug.cgi?id=744100

GNOME/json-glib,ebassi/json-glib,GNOME/json-glib,frida/json-glib,ebassi/json-glib,ebassi/json-glib,frida/json-glib