8305206: Add @spec tags in java.base/java.* (part 1)

Reviewed-by: alanb, naoto, darcy, lancea, dfuchs, iris, mchung

src/java.base/share/classes/java/nio/charset/Charset.java

@@ -262,6 +262,13 @@ import java.util.TreeMap;
  * of chars) and sequences of bytes. </p>
  *
  *
+ * @spec http://www.iana.org/assignments/character-sets Character Sets
+ * @spec https://www.rfc-editor.org/info/rfc2278
+ *      RFC 2278: IANA Charset Registration Procedures
+ * @spec https://www.rfc-editor.org/info/rfc2279
+ *      RFC 2279: UTF-8, a transformation format of ISO 10646
+ * @spec https://www.rfc-editor.org/info/rfc2781
+ *      RFC 2781: UTF-16, an encoding of ISO 10646
  * @author Mark Reinhold
  * @author JSR-51 Expert Group
  * @since 1.4
@@ -741,6 +748,7 @@ public abstract class Charset
      *
      * @return  {@code true} if, and only if, this charset is known by its
      *          implementor to be registered with the IANA
+     * @spec http://www.iana.org/assignments/character-sets Character Sets
      */
     public final boolean isRegistered() {
         return !name.startsWith("X-") && !name.startsWith("x-");