sd-json.h: reword SD_JSON_WARNING/SD_JSON_DEBUG comments

sd-json.h: reword SD_JSON_WARNING/SD_JSON_DEBUG comments

Even though we don't export json_log() in the public API, let's
officially make the SD_JSON_WARNING/SD_JSON_DEBUG that control its
effect in the public API.

After all, for our own dispatcher functions they have a nice effect, and
they are trivially reimplemented in user code independently.

(We might eventually consider exporting json_log() as public API, but
this is quite involved, given its use of macros/inline functions and
iternal logging API).

This mostly just swaps around the bit flags and cleans up comments.