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.

googlemock/src/gmock.cc

@@ -34,8 +34,8 @@
 namespace testing {
 
 GMOCK_DEFINE_bool_(catch_leaked_mocks, true,
-                   "true if Google Mock should report leaked mock objects "
-                   "as failures.");
+                   "true if and only if Google Mock should report leaked "
+                   "mock objects as failures.");
 
 GMOCK_DEFINE_string_(verbose, internal::kWarningVerbosity,
                      "Controls how verbose Google Mock's output is."