Tweak wording on availability

Author: gmlueck

sycl/doc/extensions/proposed/sycl_ext_oneapi_device_image_backend_content.asciidoc

@@ -99,6 +99,7 @@ template <bundle_state State>
 class device_image {
  public:
   std::vector<std::byte> ext_oneapi_get_backend_content() const;
+
 #if defined(__cpp_lib_span)
   std::span<std::byte> ext_oneapi_get_backend_content_view() const;
 #endif
@@ -137,7 +138,8 @@ std::span<std::byte> ext_oneapi_get_content_backend_view() const;
 ----
 !====
 
-Available only when the compiler is {cpp}20 or higher.
+Available only when the compiler defines the `__cpp_lib_span` feature-test macro
+(which is defined in {cpp}20 and higher).
 
 _Constraints:_ Available only when `State` is `bundle_state::executable`.