triagebot: add doc backlink for [no-mentions]

Author: jieyouxu

src/doc/rustc-dev-guide/triagebot.toml

@@ -55,6 +55,8 @@ allow-unauthenticated = [
 # Documentation at: https://forge.rust-lang.org/triagebot/note.html
 [note]
 
+# Prevents mentions in commits to avoid users being spammed
+# Documentation at: https://forge.rust-lang.org/triagebot/no-mentions.html
 [no-mentions]
 
 [canonicalize-issue-links]