Java: Clarify the meaning of getRuleExpression/Statement

Author: smowton

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

@@ -438,11 +438,17 @@ class SwitchCase extends Stmt, @case {
 
   /**
    * Gets the expression on the right-hand side of the arrow, if any.
+   *
+   * Note this is mutually exclusive with `getRuleStatement`: it gets a value
+   * when this case is of the form `case e1 -> e2`, where `e2` is not a block.
    */
   Expr getRuleExpression() { result.getParent() = this and result.getIndex() = -1 }
 
   /**
    * Gets the statement on the right-hand side of the arrow, if any.
+   *
+   * Note this is mutually exclusive with `getRuleExpression`: it gets a value
+   * when this case if of the form `case e1 -> { s1; s2; ... }`.
    */
   Stmt getRuleStatement() { result.getParent() = this and result.getIndex() = -1 }
 }