Document new allowrcpt restriction on adding user email

Since change Ic83e1f739 the new email address must belong to a
domain that is allowed by the sendemail.allowrcpt setting.

Update the documentation to mention this.

Change-Id: Ib818535b0fa30dd25fde2d92837bfa913cb71f9e
diff --git a/Documentation/rest-api-accounts.txt b/Documentation/rest-api-accounts.txt
index 7aec531..a510e8a 100644
--- a/Documentation/rest-api-accounts.txt
+++ b/Documentation/rest-api-accounts.txt
@@ -442,6 +442,9 @@
 confirmation. A Gerrit administrator may add an email address without
 confirmation by setting `no_confirmation` in the
 link:#email-input[EmailInput].
+If link:config-gerrit.html#sendemail.allowrcpt[sendemail.allowrcpt] is
+configured, the added email address must belong to a domain that is
+allowed, unless `no_confirmation` is set.
 
 In the request body additional data for the email address can be
 provided as link:#email-input[EmailInput].