Add JSON error code 40006

[evnsh]

Also edited the example JSON error as the code is always after the error message

[Lachee]

Should mention that order of json fields shouldn't be relied on. It seems like code was written on top of the message originally for stylistic purposes.

[msciotti]

For those interested in the usage of this, at the time of merging we use this in two places:

1. A guild trying to enable the public flag/server discovery from our API when they are not eligible

2. Trying to add reactions on a message in a server on which we've disabled reactions for performance reasons.

Number 2 happens intermittently.