enum documentation not updated for RFC 2195

[RalfJung]

https://doc.rust-lang.org/reference/items/enumerations.html#custom-discriminant-values-for-field-less-enumerations talks about "Custom Discriminant Values for Field-Less Enumerations", but does not mention at all that since https://github.com/rust-lang/rfcs/blob/master/text/2195-really-tagged-unions.md Rust also supports custom discriminant values and defined layout for enums with fields.

[ehuss]

It looks like there was an attempt to do this at #246 that wasn't completed.

[Havvy]

cc-ing @poliorcetics as requested in Zulip.

[poliorcetics]

Opened #879 to hopefully fix this.