restore mistakenly removed iffs in their explicit form

Due to confusion arisen from "iff" standing for "if and only if",
this commit uses the latter.

googletest/samples/prime_tables.h

@@ -43,7 +43,7 @@ class PrimeTable {
  public:
   virtual ~PrimeTable() {}
 
-  // Returns true if n is a prime number.
+  // Returns true if and only if n is a prime number.
   virtual bool IsPrime(int n) const = 0;
 
   // Returns the smallest prime number greater than p; or returns -1

googletest/samples/sample1.cc

@@ -41,7 +41,7 @@ int Factorial(int n) {
   return result;
 }
 
-// Returns true if n is a prime number.
+// Returns true if and only if n is a prime number.
 bool IsPrime(int n) {
   // Trivial case 1: small numbers
   if (n <= 1) return false;

googletest/samples/sample1.h

@@ -35,7 +35,7 @@
 // Returns n! (the factorial of n).  For negative n, n! is defined to be 1.
 int Factorial(int n);
 
-// Returns true if n is a prime number.
+// Returns true if and only if n is a prime number.
 bool IsPrime(int n);
 
 #endif  // GTEST_SAMPLES_SAMPLE1_H_