* remove tip in docs about SocketEntity.Discord
* Expose the 'Fields' collection on EmbedBuilder
After some discussion I decided that there was really no reason to keep this private, and it didn't really go along with the rest of the design of the EmbedBuilder.
This is NOT a breaking change.
Exposing this property should not have any negative effects.
* Don't allow EmbedBuilder's Fields to be set to null
