diff --git a/doc/manual/command-ref/nix-env.xml b/doc/manual/command-ref/nix-env.xml
index 9d13d3014..c8c01f9a4 100644
--- a/doc/manual/command-ref/nix-env.xml
+++ b/doc/manual/command-ref/nix-env.xml
@@ -1371,7 +1371,10 @@ special value <literal>old</literal> to delete all non-current
 generations,  a value such as <literal>30d</literal> to delete all
 generations older than the specified number of days (except for the
 generation that was active at that point in time), or a value such as
-<literal>+5</literal> to only keep the specified number of generations.
+<literal>+5</literal> to keep the last <literal>5</literal> generations
+ignoring any newer than current, e.g., if <literal>30</literal> is the current
+generation <literal>+5</literal> will delete generation <literal>25</literal>
+and all older generations.
 Periodically deleting old generations is important to make garbage collection
 effective.</para>