diff --git a/documentation/ref-manual/variables.rst b/documentation/ref-manual/variables.rst
index cb082cbdbc..0d2d255edb 100644
--- a/documentation/ref-manual/variables.rst
+++ b/documentation/ref-manual/variables.rst
@@ -6878,6 +6878,11 @@ system and gives an overview of their function and contents.
       service <dev-manual/packages:working with a pr service>`. You can
       set :term:`PRSERV_HOST` to other values to use a remote PR service.
 
+   :term:`PRSERV_UPSTREAM`
+      This variable can be used to specify an upstream PR server for the local
+      PR server to connect to, in the form of ``host:port``.
+
+      This makes it possible to implement local fixes to an upstream package.
 
    :term:`PSEUDO_IGNORE_PATHS`
       A comma-separated (without spaces) list of path prefixes that should be ignored