Consolidate label descriptions in issue triage chapter and rustc dev guide

[Noratrieb]

There is a section in the rustc dev guide that covers this. Should probably consolidate these into a single place with a link from the either.

Originally posted by @jackh726 in #716 (comment)

[jieyouxu]

Actually this was resolved in Only keep issue triage docs in Forge docs rust-lang/rustc-dev-guide#2194 and #794