Document rustc_middle::lint::LevelSource

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

Appease x.py fmt.

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 {