doc: document dl-json

[wc-duck]

DL uses json... isch ;) Document the difference.

• Valid json IS valid dl-json
• DL-json support comments // and /**/
• DL json can use '' or "" for strings/keys
• in dl-json '' or "" is optional on keys
• Keys need to be valid c-identifiers
• , is accepted for last item in array and map
• new-line in strings are valid, or 'my_str' : "whoo Whaaa Whiiii"

[RandyGaul]

What is map?

[wc-duck]

A {} in json