Swift: Add qldoc for BinaryExpr.

Author: geoffw0

swift/ql/lib/codeql/swift/elements/expr/BinaryExpr.qll

@@ -2,13 +2,32 @@ private import codeql.swift.generated.expr.BinaryExpr
 private import codeql.swift.elements.expr.Expr
 private import codeql.swift.elements.decl.AbstractFunctionDecl
 
+/**
+ * A Swift binary expression, that is, an expression that appears between its
+ * two operands. For example:
+ * ```
+ * x + y
+ * ```
+ */
 class BinaryExpr extends Generated::BinaryExpr {
+  /**
+   * Gets the left operand (left expression) of this binary expression.
+   */
   Expr getLeftOperand() { result = this.getArgument(0).getExpr() }
 
+  /**
+   * Gets the right operand (right expression) of this binary expression.
+   */
   Expr getRightOperand() { result = this.getArgument(1).getExpr() }
 
+  /**
+   * Gets the operator of this binary expression (the function that is called).
+   */
   AbstractFunctionDecl getOperator() { result = this.getStaticTarget() }
 
+  /**
+   * Gets an operand of this binary expression (left or right).
+   */
   Expr getAnOperand() { result = [this.getLeftOperand(), this.getRightOperand()] }
 
   override string toString() { result = "... " + this.getFunction().toString() + " ..." }