Java: Add qldoc.

java/ql/src/semmle/code/Unit.qll

@@ -5,5 +5,6 @@ private newtype TUnit = TMkUnit()
 
 /** The trivial type with a single element. */
 class Unit extends TUnit {
+  /** Gets a textual representation of this element. */
   string toString() { result = "unit" }
 }