Document integer enum discriminators in the manual

[mbrubeck]

The tutorial documents how to set the discriminator for a C-style enum (enum Foo { Bar = 123 }) and how to cast such an enum to a numeric type (Bar as uint). However, the language manual does not mention these features at all.

[dcampbell24]

I think it would be good to also mention what the type of the discriminant is and how to control it per #9613.