docs: add environment variables for feed configuration

Author: voronkovich

docs/reference/configuration.md

@@ -1047,12 +1047,16 @@ The number of content items to be shown in any RSS news feeds set up on the webs
 
 Default: `10`
 
+Env: `JOOMLA_SITE_FEED_LIMIT`
+
 ### feed_email
 
 Feed email: `none`, `author`, `site`.
 
 Default: `none`
 
+Env: `JOOMLE_SITE_FEED_EMAIL`
+
 ### Example
 
 ```php title="configuration.php"
@@ -1093,6 +1097,6 @@ JOOMLA_SITE_OFFLINE_MESSAGE=Maintenance
 JOOMLA_SITE_OFFLINE_MESSAGE_DISPLAY=2
 JOOMLA_SITE_OFFLINE_IMAGE=images/offline.png
 
-# feed_limit is not available as an environment variable
-# feed_email is not available as an environment variable
+JOOMLA_SITE_FEED_LIMIT=20
+JOOMLE_SITE_FEED_EMAIL=author
 ```