Document ilias web crawler's forums option

CHANGELOG.md

@@ -22,6 +22,9 @@ ambiguous situations.
 
 ## Unreleased
 
+### Added
+- Missing documentation for `forums` option
+
 ### Changed
 - Clear up error message shown when multiple paths are found to an element
 

CONFIG.md

@@ -181,6 +181,7 @@ script once per day should be fine.
   redirect to the actual URL. Set to a negative value to disable the automatic
   redirect. (Default: `-1`)
 - `videos`: Whether to download videos. (Default: `no`)
+- `forums`: Whether to download forum threads. (Default: `no`)
 - `http_timeout`: The timeout (in seconds) for all HTTP requests. (Default:
   `20.0`)