doc: a follow-up fix for commit 82e1673.

agentzh · agentzh · commit fa71ba617040 · 2016-02-10T09:56:11.000-08:00
diff --git a/README.markdown b/README.markdown
@@ -843,7 +843,8 @@ The `<time>` argument values are in seconds by default. But it's wise to always
 
 This time must be less than 597 hours.
 
-Setting an expire time of 0s will cache the content forever.
+The semantics of a zero expiration time depends on the actual cache backend storage you are currently using, which is agnostic to this
+module. In the case of memcached, for example, zero expiration times mean that the item will never expire.
 
 This directive was first introduced in the `v0.12rc7` release.
 
