Tests/RunCMake: Document stripping of expected output

Document that trailing newlines are stripped from the -stdout.txt
and -stderr.txt content before using as a regular expression.

Tests/RunCMake/CMakeLists.txt

@@ -28,7 +28,7 @@
 #   <SubTest>-stdout.txt  = Regex matching expected stdout content
 #   <SubTest>-stderr.txt  = Regex matching expected stderr content
 #   <SubTest>-check.cmake = Custom result check
-# Note that trailing newlines will be stripped from actual test
+# Note that trailing newlines will be stripped from actual and expected test
 # output before matching against the stdout and stderr expressions.
 # The code in <SubTest>-check.cmake may use variables
 #   RunCMake_TEST_SOURCE_DIR = Top of test source tree