Link to allowRegisterNewEmail setting from warning in config.md

If access to the REST API is enabled, service users can use the Create
Email REST endpoint in Gerrit core to create an email address even if
'allowEmail' is set to 'false'. This is not the case if
'sendemail.allowRegisterNewEmail' in the gerrit.config is set to
'false'.

Change-Id: Ib1880cc64c2574bbd78d08a0f23f92812327aeb7
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
diff --git a/src/main/resources/Documentation/config.md b/src/main/resources/Documentation/config.md
index 1d5bdb2..8ca7754 100644
--- a/src/main/resources/Documentation/config.md
+++ b/src/main/resources/Documentation/config.md
@@ -109,7 +109,8 @@
 *WARNING*: If access to the REST API is enabled, service users can use
 the [Create Email REST endpoint](../../../Documentation/rest-api-accounts.html#create-account-email)
 in Gerrit core to create an email address even if
-[allowEmail](#allowEmail) is set to `false`.
+[allowEmail](#allowEmail) is set to `false`, unless
+[registration of email addresses in Gerrit is disabled](../../../Documentation/config-gerrit.html#sendemail.allowRegisterNewEmail).
 
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