fixup: expand the diagnostic message

Author: a-nogikh

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3450,8 +3450,9 @@ def warn_attribute_return_pointers_only : Warning<
   "%0 attribute only applies to return values that are pointers">,
   InGroup<IgnoredAttributes>;
 def warn_attribute_return_span_only
-    : Warning<"%0 attribute only applies to return values that are span-like "
-              "structures">,
+    : Warning<"%0 attribute only applies to functions that return span-like structures: "
+    "one field is a pointer to the allocated memory and another field is an integer with "
+    "the size of the allocated memory">,
       InGroup<IgnoredAttributes>;
 def warn_attribute_return_pointers_refs_only : Warning<
   "%0 attribute only applies to return values that are pointers or references">,

clang/test/Sema/attr-malloc_span.c

@@ -1,7 +1,7 @@
 // RUN: %clang_cc1 -verify -fsyntax-only %s
 // RUN: %clang_cc1 -emit-llvm -o %t %s
 
-#include <stddef.h>
+typedef __SIZE_TYPE__ size_t;
 
 typedef struct {
   void *ptr;
@@ -21,11 +21,11 @@ typedef struct {
   void *ptr;
   void *ptr2;
 } invalid_span1;
-invalid_span1  returns_non_std_span1  (void) __attribute((malloc_span)); // expected-warning {{attribute only applies to return values that are span-like structures}}
+invalid_span1  returns_non_std_span1  (void) __attribute((malloc_span)); // expected-warning {{attribute only applies to functions that return span-like structures}}
 
 typedef struct {
   void *ptr;
   size_t n;
   size_t n2;
 } invalid_span2;
-invalid_span2  returns_non_std_span2  (void) __attribute((malloc_span)); // expected-warning {{attribute only applies to return values that are span-like structures}}
+invalid_span2  returns_non_std_span2  (void) __attribute((malloc_span)); // expected-warning {{attribute only applies to functions that return span-like structures}}