Add rax cache age ini documentation

contrib/inventory/rax.ini

@@ -55,3 +55,12 @@
 # will be ignored, and 4 will be used. Accepts a comma separated list,
 # the first found wins.
 # access_ip_version = 4
+
+# Environment Variable: RAX_CACHE_MAX_AGE
+# Default: 600
+#
+# A configuration the changes the behavior or the inventory cache.
+# Inventory listing performed before this value will be returned from
+# the cache instead of making a full request for all inventory. Setting
+# this value to 0 will force a full request.
+# cache_max_age = 600