Unified: Add qldoc.

Author: aschackmull

unified/ql/lib/codeql/unified/internal/FacadeAst.qll

@@ -51,6 +51,7 @@ module Unified {
 
   /** A binary expression. */
   class BinaryExpr extends G::BinaryExpr {
+    /** Gets an operand of this binary expression. */
     Expr getAnOperand() { result = [this.getLeft(), this.getRight()] }
   }