Add clarifying comments about when to use EXPECT_NONFATAL_FAILURE vs. EXPECT_FATAL_FAILURE.

PiperOrigin-RevId: 448237839
Change-Id: Id6242f278912f8c47cf19e3ea8c061f8b18ba832

googletest/include/gtest/gtest-spi.h

@@ -121,7 +121,8 @@ class GTEST_API_ SingleFailureChecker {
 GTEST_DISABLE_MSC_WARNINGS_POP_()  //  4251
 
 // A set of macros for testing Google Test assertions or code that's expected
-// to generate Google Test fatal failures.  It verifies that the given
+// to generate Google Test fatal failures (e.g. a failure from an ASSERT_EQ, but
+// not a non-fatal failure, as from EXPECT_EQ).  It verifies that the given
 // statement will cause exactly one fatal Google Test failure with 'substr'
 // being part of the failure message.
 //
@@ -179,9 +180,10 @@ GTEST_DISABLE_MSC_WARNINGS_POP_()  //  4251
   } while (::testing::internal::AlwaysFalse())
 
 // A macro for testing Google Test assertions or code that's expected to
-// generate Google Test non-fatal failures.  It asserts that the given
-// statement will cause exactly one non-fatal Google Test failure with 'substr'
-// being part of the failure message.
+// generate Google Test non-fatal failures (e.g. a failure from an EXPECT_EQ,
+// but not from an ASSERT_EQ). It asserts that the given statement will cause
+// exactly one non-fatal Google Test failure with 'substr' being part of the
+// failure message.
 //
 // There are two different versions of this macro. EXPECT_NONFATAL_FAILURE only
 // affects and considers failures generated in the current thread and