Document EmailArguments

While adding the javadoc I was wondering why this class is not a
singleton. Adding a todo to check this later.

Change-Id: Ie7639e2d4fff62972873e4c6457e7d00f3606d84
Signed-off-by: Edwin Kempin <ekempin@google.com>

java/com/google/gerrit/server/mail/send/EmailArguments.java

@@ -50,6 +50,19 @@ import java.util.List;
 import org.eclipse.jgit.lib.Config;
 import org.eclipse.jgit.lib.PersonIdent;
 
+/**
+ * Arguments used for sending notification emails.
+ *
+ * <p>Notification emails are sent by out by {@link OutgoingEmail} and it's subclasses, so called
+ * senders. To construct an email the sender class needs to get various other classes injected.
+ * Instead of injecting these classes into the sender classes directly, they only get {@code
+ * EmailArguments} injected and {@code EmailArguments} provides them all dependencies that they
+ * need.
+ *
+ * <p>This class is public because plugins need access to it for sending emails.
+ */
+// TODO(ekempin): Can we make this class a singleton so that we don't need to instantiate it for
+// each and every email?
 @UsedAt(UsedAt.Project.PLUGINS_ALL)
 public class EmailArguments {
   final GitRepositoryManager server;