Document rustc_middle::lint::LevelSource

This is to clarify the difference between `LevelSource`
and `LintLevelSource`.

Appease x.py fmt.

compiler/rustc_lint/src/levels.rs

@@ -12,7 +12,9 @@ use rustc_hir::{intravisit, HirId};
 use rustc_middle::hir::map::Map;
 use rustc_middle::lint::LevelSource;
 use rustc_middle::lint::LintDiagnosticBuilder;
-use rustc_middle::lint::{struct_lint_level, LintLevelMap, LintLevelSets, LintSet, LintLevelSource};
+use rustc_middle::lint::{
+    struct_lint_level, LintLevelMap, LintLevelSets, LintLevelSource, LintSet,
+};
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::TyCtxt;
 use rustc_session::lint::{builtin, Level, Lint, LintId};

compiler/rustc_middle/src/lint.rs

@@ -45,6 +45,7 @@ impl LintLevelSource {
     }
 }
 
+/// A tuple of a lint level and its source.
 pub type LevelSource = (Level, LintLevelSource);
 
 pub struct LintLevelSets {