Unsupported is unclear.

config/application.rb

@@ -70,7 +70,7 @@ module Gitlab
     config.assets.version = '1.0'
 
     # Uncomment and customize the last line to run in a non-root path
-    # WARNING: This feature is known to work, but unsupported
+    # WARNING: We recommend creating a FQDN to host GitLab in a root path.
     # Note that three settings need to be changed for this to work.
     # 1) In your application.rb file: config.relative_url_root = "/gitlab"
     # 2) In your gitlab.yml file: relative_url_root: /gitlab

config/gitlab.yml.example

@@ -20,7 +20,7 @@ production: &base
     https: false
 
     # Uncomment and customize the last line to run in a non-root path
-    # WARNING: This feature is known to work, but unsupported
+    # We recommend creating a FQDN to host GitLab in a root path.
     # Note that three settings need to be changed for this to work.
     # 1) In your application.rb file: config.relative_url_root = "/gitlab"
     # 2) In your gitlab.yml file: relative_url_root: /gitlab

config/unicorn.rb.example

@@ -9,7 +9,7 @@
 # documentation.
 
 # Uncomment and customize the last line to run in a non-root path
-# WARNING: This feature is known to work, but unsupported
+# We recommend creating a FQDN to host GitLab in a root path.
 # Note that three settings need to be changed for this to work.
 # 1) In your application.rb file: config.relative_url_root = "/gitlab"
 # 2) In your gitlab.yml file: relative_url_root: /gitlab