Add missing QLDocs

Author: owen-mc

java/ql/lib/semmle/code/java/ControlFlowGraph.qll

@@ -152,6 +152,7 @@ module ControlFlow {
     ExprParent getAstNode() { none() }
   }
 
+  /** A control-flow node that represents the evaluation of an expression. */
   class ExprNode extends Node, TExprNode {
     Expr e;
 
@@ -174,6 +175,7 @@ module ControlFlow {
     override Location getLocation() { result = e.getLocation() }
   }
 
+  /** A control-flow node that represents a statement. */
   class StmtNode extends Node, TStmtNode {
     Stmt s;
 
@@ -194,7 +196,7 @@ module ControlFlow {
     override Location getLocation() { result = s.getLocation() }
   }
 
-  /** A synthetic node for the exit of a callable. */
+  /** A synthetic control-flow node for the exit of a callable. */
   class ExitNode extends Node, TExitNode {
     Callable c;