[analyzer] CStringChecker: Improve warning messages.

Differential Revision: https://reviews.llvm.org/D71321
diff --git a/clang/test/Analysis/bsd-string.c b/clang/test/Analysis/bsd-string.c
index d8a8883..3778664 100644
--- a/clang/test/Analysis/bsd-string.c
+++ b/clang/test/Analysis/bsd-string.c
@@ -33,11 +33,11 @@
 }
 
 void f4() {
-  strlcpy(NULL, "abcdef", 6); // expected-warning{{Null pointer argument in call to string copy function}}
+  strlcpy(NULL, "abcdef", 6); // expected-warning{{Null pointer passed as 1st argument to string copy function}}
 }
 
 void f5() {
-  strlcat(NULL, "abcdef", 6); // expected-warning{{Null pointer argument in call to string copy function}}
+  strlcat(NULL, "abcdef", 6); // expected-warning{{Null pointer passed as 1st argument to string concatenation function}}
 }
 
 void f6() {