message: Clarify the meaning of feature-related labels
diff: @@ -147,6 +147,11 @@ adding labels to triage issues:
  * Magenta, **B**-prefixed labels identify bugs which are **blockers**.
  * Light purple, **C**-prefixed labels represent the **category** of an issue.
  + In particular, **C-feature-request** marks *proposals* for new features. If
  + an issue is **C-feature-request**, but is not **Feature accepted** or **I-nominated**,
  + then it was not thoroughly discussed, and might need some additional design
  + or perhaps should be implemented as an external subcommand first. Ping
  + @rust-lang/cargo if you want to send a PR for such issue.
  * Dark purple, **Command**-prefixed labels mean the issue has to do with a
  specific cargo command.
  