Add documentation about required value

Author: holly-cummins

docs/src/main/asciidoc/mailer-reference.adoc

@@ -66,9 +66,9 @@ To send a simple email, proceed as follows:
 [source, java]
 ----
 // Imperative API:
-mailer.send(Mail.withText("[email protected]", "A simple email from quarkus", "This is my body."));
+mailer.send(Mail.withText("[email protected]", "A simple email from quarkus", "This is my body.").setFrom("[email protected]"));
 // Reactive API:
-Uni<Void> stage = reactiveMailer.send(Mail.withText("[email protected]", "A reactive email from quarkus", "This is my body."));
+Uni<Void> stage = reactiveMailer.send(Mail.withText("[email protected]", "A reactive email from quarkus", "This is my body.").setFrom("[email protected]"));
 ----
 
 For example, you can use the `Mailer` in an HTTP endpoint as follows:
@@ -78,14 +78,14 @@ For example, you can use the `Mailer` in an HTTP endpoint as follows:
 @GET
 @Path("/imperative")
 public void sendASimpleEmail() {
-    mailer.send(Mail.withText("[email protected]", "A simple email from quarkus", "This is my body"));
+    mailer.send(Mail.withText("[email protected]", "A simple email from quarkus", "This is my body").setFrom("[email protected]"));
 }
 
 @GET
 @Path("/reactive")
 public Uni<Void> sendASimpleEmailAsync() {
     return reactiveMailer.send(
-            Mail.withText("[email protected]", "A reactive email from quarkus", "This is my body"));
+            Mail.withText("[email protected]", "A reactive email from quarkus", "This is my body").setFrom("[email protected]"));
 }
 ----
 
@@ -96,6 +96,20 @@ You can create new `io.quarkus.mailer.Mail` instances from the constructor or fr
 `Mail.withHtml` helper methods.
 The `Mail` instance lets you add recipients (to, cc, or bcc), set the subject, headers, sender (from) address...
 
+Most of these properties are optional, but the sender address is required. It can either be set on an individual `Mail` instances using
+
+[source,java]
+----
+.setFrom("[email protected]");
+----
+
+or a global default can be configured, using
+
+[source,properties]
+----
+[email protected]
+----
+
 You can also send several `Mail` objects in one call:
 
 [source, java]