json-node: Clarify documentation regarding programmer errors

Despite my efforts in commit 011c1e42 (parts of which were completely incorrect), the JsonNode documentation still wasn’t particularly helpful about differentiating between methods which return NULL if the JsonNode is not of the right type, and methods which emit a critical warning.

Try and improve that across the board, hopefully without saying blatantly incorrect things this time.

Signed-off-by: Philip Withnall withnall@endlessm.com