Update 3k_users.md

doc/administration/reference_architectures/3k_users.md

@@ -17,6 +17,8 @@ highly-available GitLab deployment. If you do not have the expertise or need to
 maintain a highly-available environment, you can have a simpler and less
 costly-to-operate environment by using the
 [2,000-user reference architecture](2k_users.md).
+If you have fewer than 3,000 users and still want a highly-available GitLab deployment,
+you should still use this reference architecture but scale down the node sizes.
 
 > - **Supported users (approximate):** 3,000
 > - **High Availability:** Yes