core: clarify security methods on channel and server

core/src/main/java/io/grpc/ManagedChannelBuilder.java

@@ -152,6 +152,8 @@ public abstract class ManagedChannelBuilder<T extends ManagedChannelBuilder<T>>
    *
    * @param skipNegotiation @{code true} if there is a priori knowledge that the endpoint supports
    *                        plaintext, {@code false} if plaintext use must be negotiated.
+   *
+   * @throws UnsupportedOperationException if plaintext mode is not supported.
    * @return this
    * @since 1.0.0
    */

core/src/main/java/io/grpc/ServerBuilder.java

@@ -145,6 +145,7 @@ public abstract class ServerBuilder<T extends ServerBuilder<T>> {
    * @param privateKey file containing the private key
    *
    * @return this
+   * @throws UnsupportedOperationException if the server does not support TLS.
    * @since 1.0.0
    */
   public abstract T useTransportSecurity(File certChain, File privateKey);