Manual:$wgShowUpdatedMarker

Details
If watchlisted page was changed after it was visited:


 * history page: new edits are marked with ""


 * watchlist: the page title is bolded

Also the watchlist has a message "" and a button "" that marks all entries as "visited".

If enabled, action=info will display the number of active watchers of the page based on $wgWatchersMaxAge.

Dependencies
If   is false, this setting is ignored.

Styling
For different styling of watchlist entries see w:WP:Customizing watchlists.