Add hints about usage of deprecated methods

pgpainless-core/src/main/java/org/pgpainless/decryption_verification/ConsumerOptions.java

@@ -287,6 +287,7 @@ public class ConsumerOptions {
      * @return options
      */
     @Deprecated
+    // Is used to be compatible with legacy systems
     public ConsumerOptions setIgnoreMDCErrors(boolean ignoreMDCErrors) {
         this.ignoreMDCErrors = ignoreMDCErrors;
         return this;

pgpainless-core/src/main/java/org/pgpainless/encryption_signing/ProducerOptions.java

@@ -245,6 +245,7 @@ public final class ProducerOptions {
      * @deprecated options other than the default value of {@link StreamEncoding#BINARY} are discouraged.
      */
     @Deprecated
+    // is used to be compatible with legacy systems
     public ProducerOptions setEncoding(@Nonnull StreamEncoding encoding) {
         this.encodingField = encoding;
         return this;