[docs] Build with JavaDoc 17 and add missing docs (#6220)

Co-authored-by: Sam Carlberg <sam.carlberg@gmail.com>

wpiunits/src/main/java/edu/wpi/first/units/collections/LongToObjectHashMap.java

@@ -225,8 +225,19 @@ public class LongToObjectHashMap<V> {
     return List.copyOf(values); // return a readonly copy
   }
 
+  /**
+   * Interface for map iterator function.
+   *
+   * @param <V> Value type.
+   */
   @FunctionalInterface
   public interface IteratorFunction<V> {
+    /**
+     * Accepts a key-value pair from the map.
+     *
+     * @param key The key.
+     * @param value The value.
+     */
     void accept(long key, V value);
   }