json: Rework verify_result_to_json

* src/gpgme-json.c (sigsum_to_json): Add bool repr.
(signature_to_json, verify_result_to_json): Extend and follow better
pattern.
(hlp_decrypt, hlp_verify): Expand doc.

This should make it more clear which values are mapped as
the naming is more direct and clear and help to use
the gpgme documentation to understand the verify_result values.