fix: comment refactoring

google-cloud-spanner/src/main/java/com/google/cloud/spanner/connection/ClientSideStatementValueConverters.java

@@ -501,7 +501,7 @@ public String convert(String value) {
     }
   }
 
-  /** Converter for converting strings to byte[] */
+  /** Converter for converting Base64 encoded string to byte[] */
   static class ProtoDescriptorsConverter implements ClientSideStatementValueConverter<byte[]> {
 
     public ProtoDescriptorsConverter(String allowedValues) {}