Rollup merge of #61056 - euclio:custom-discriminant-error, r=estebank

tweak discriminant on non-nullary enum diagnostic

Adds notes pointing at the non-nullary variants, and uses "custom
discriminant" language to be consistent with the Reference.

Fixes #61039.

r? @estebank

diff --cc src/libsyntax/parse/parser.rs
Simple merge