Clarify archiving/purge documentation (#9135)

* Clarify archiving/purge documentation

Explain purge policy priority (feed > category > global), the discovery-date based purge logic, and that purging is only triggered on a random subset of feed refreshes (or via Purge now / cli/purge.php).

Fixes #6863.

Co-Authored-By: Claude <noreply@anthropic.com>

* Typographic quotes

* No changelog for (minor) documentation

---------

Co-authored-by: Gerard Alvear <gerard.alvear@logiqd.me>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Alexandre Alapetite <alexandre@alapetite.fr>

docs/en/users/04_Subscriptions.md

@@ -21,7 +21,9 @@ The "Subscription management" submenu allows categories and feeds to be configur
 
 ### Archiving
 
-If "Purge Policy" has "By default" selected, then the [default purge policy](./05_Configuration.md) is used and the other options are not displayed. Category options will override the default policy, but they will not override feed-specific options.
+If “Purge Policy” has “By default” selected, then the [default purge policy](./05_Configuration.md#archiving) is used and the other options are not displayed. Category options will override the default policy, but they will not override feed-specific options.
+
+See [Archiving](./05_Configuration.md#archiving) for important details on when and how purging actually happens (it is not applied on every refresh, and it is based on the discovery date of articles, not their published date).
 
 ## Feed Settings
 
@@ -29,7 +31,7 @@ These fields will be auto-filled when adding a feed, but they can be modified la
 
 ### Archival
 
-This section will let you override the default settings for feed archiving and update frequency.
+This section will let you override the default settings for feed archiving and update frequency. When “By default” is selected here, the feed uses its category’s purge policy, falling back to the [global default](./05_Configuration.md#archiving) if the category also uses the default.
 
 ### Login
 

docs/en/users/05_Configuration.md

@@ -129,11 +129,15 @@ This will set images to load as they are viewed. This can save data, but will ca
 
 ### Archiving
 
-These are the global options for fetching and retaining articles from feeds. They can be overridden by individual feed’s settings.
+These are the global options for fetching and retaining articles from feeds. They can be overridden by [category](./04_Subscriptions.md#archiving) settings, which can in turn be overridden by individual feed’s settings (priority order: feed, then category, then this global default).
+
+Purging (i.e. deleting) an article is based on the date it was _discovered_ by FreshRSS, not its published date. An article still present in the upstream feed is always kept regardless of the purge policy, so FreshRSS can keep detecting new articles correctly.
+
+Purging does not happen on every feed refresh: it is only triggered on a random subset of refreshes, to limit the load on the database. To purge immediately, use _Purge now_ below, or the `cli/purge.php` [command-line script](https://github.com/FreshRSS/FreshRSS/blob/edge/cli/README.md), which is suitable for cron.
 
 ### Maintenance
 
-This allows for purging/optimizing the current user’s articles in the database.
+This allows for purging/optimizing the current user’s articles in the database. _Purge now_ immediately applies the archiving policy (feed, then category, then global default) described above to all of your feeds.
 
 ## Sharing