doc: tweak documention of the longLine linter (#15458)

Mathlib/Tactic/Linter/Lint.lean

@@ -219,6 +219,8 @@ initialize addLinter cdotLinter
 
 end CDotLinter
 
+/-! # The "longLine linter" -/
+
 /-- The "longLine" linter emits a warning on lines longer than 100 characters.
 We allow lines containing URLs to be longer, though. -/
 register_option linter.longLine : Bool := {
@@ -237,8 +239,7 @@ def longLineLinter : Linter where run := withSetOptionIn fun stx ↦ do
       return
     if (← MonadState.get).messages.hasErrors then
       return
-    -- TODO: once per-project settings are available,
-    -- revert this hack to make it only apply on `Mathlib`
+    -- TODO: once mathlib's Lean version includes leanprover/lean4#4741, make this configurable
     unless #[`Mathlib, `test, `Archive, `Counterexamples].contains (← getMainModule).getRoot do
       return
     -- The linter ignores the `#guard_msgs` command, in particular its doc-string.