Clarify that SmtpEmailSender doesn't support attachments.

The SmtpEmailSender is old, builds emails in a not particular structured
manner and has no existing test coverage. So while implementation
wouldn't be difficult, for now it's not going to support inline resource
attachments.

Google-Bug-Id: b/283758650
Release-Notes: skip
Change-Id: I85c36a85851f3ba6bcefba1f5cce96ac1cb21830
diff --git a/java/com/google/gerrit/server/mail/send/SmtpEmailSender.java b/java/com/google/gerrit/server/mail/send/SmtpEmailSender.java
index c06cc1e..98a986c 100644
--- a/java/com/google/gerrit/server/mail/send/SmtpEmailSender.java
+++ b/java/com/google/gerrit/server/mail/send/SmtpEmailSender.java
@@ -53,7 +53,11 @@
 import org.apache.james.mime4j.codec.QuotedPrintableOutputStream;
 import org.eclipse.jgit.lib.Config;
 
-/** Sends email via a nearby SMTP server. */
+/**
+ * Sends email via a nearby SMTP server.
+ *
+ * <p>Doesn't support including EmailResource in the payload.
+ */
 @Singleton
 public class SmtpEmailSender implements EmailSender {
   /** The socket's connect timeout (0 = infinite timeout) */