4842457: (bf spec) Clarify meaning of "(optional operation)"

Reviewed-by: alanb

src/java.base/share/classes/java/nio/X-Buffer.java.template

@@ -258,6 +258,15 @@ import jdk.internal.util.ArraysSupport;
  * cb.put("text/").put(subtype).put("; charset=").put(enc);</pre></blockquote>
  *
 #end[char]
+ * <h2> Optional operations </h2>
+ * Methods specified as
+ * <i>{@linkplain Buffer##read-only-buffers-heading optional
+ * operations}</i> throw a {@linkplain ReadOnlyBufferException} when invoked
+ * on a {@linkplain java.nio.Buffer#isReadOnly read-only} $Type$Buffer. The
+ * methods {@linkplain #array array} and {@linkplain #arrayOffset arrayOffset}
+ * throw an {@linkplain UnsupportedOperationException} if the $Type$Buffer is
+ * not backed by an {@linkplain Buffer#hasArray accessible $type$ array}
+ * (irrespective of whether the $Type$Buffer is read-only).
  *
  *
  * @author Mark Reinhold