Json Value `:=` Syntax Not Documented in Help Message

[zicklag]

The current bat help message says:

...
ITEM:
  Can be any of:
    Query string   key=value
    Header         key:value
    Post data      key=value
    File upload    key@/path/file
...

I was trying to send a JSON number and thought that it was not possible until I read the readme for bat and found that it was possible using the key:=value syntax. It would be great to see that syntax documented in the help message.