Add @NonNull annotations to @NullUnmarked APIs.

While [we're not sure about the attribute
values](#10 (review)),
we believe that everything else is non-null.

I hadn't thought of using `@NonNull` back then, but it occurred to me
after I put together #109.

src/java.base/share/classes/java/nio/file/Files.java

@@ -81,6 +81,7 @@
 import java.util.stream.Stream;
 import java.util.stream.StreamSupport;
 
+import org.jspecify.annotations.NonNull;
 import org.jspecify.annotations.NullMarked;
 import org.jspecify.annotations.NullUnmarked;
 import org.jspecify.annotations.Nullable;
@@ -1996,9 +1997,9 @@ public static Path setAttribute(Path path, String attribute, Object value,
      *          to read security sensitive attributes then the security manager
      *          may be invoked to check for additional permissions.
      */
-    @NullUnmarked
-    public static Object getAttribute(Path path, String attribute,
-                                      LinkOption... options)
+    @NullUnmarked // https://github.com/jspecify/jdk/pull/10#pullrequestreview-1404320091
+    public static Object getAttribute(@NonNull Path path, @NonNull String attribute,
+                                      @NonNull LinkOption @NonNull ... options)
         throws IOException
     {
         // only one attribute should be read
@@ -2101,9 +2102,9 @@ public static Object getAttribute(Path path, String attribute,
      *          to read security sensitive attributes then the security manager
      *          may be invoked to check for additional permissions.
      */
-    @NullUnmarked
-    public static Map<String,Object> readAttributes(Path path, String attributes,
-                                                    LinkOption... options)
+    @NullUnmarked // https://github.com/jspecify/jdk/pull/10#pullrequestreview-1404320091
+    public static @NonNull Map<@NonNull String,Object> readAttributes(@NonNull Path path, @NonNull String attributes,
+                                                    @NonNull LinkOption @NonNull ... options)
         throws IOException
     {
         return provider(path).readAttributes(path, attributes, options);