Implemented in https://gerrit.wikimedia.org/r/#/c/152359/
Intended purpose or use is not clear.
Provided release note:
```
Simplified continuation will return a "batchcomplete" property in the result
when a batch of pages is complete.
```
Also, it seems to be added to responses for `action=watch`:
```
{
"batchcomplete":"",
"watch":[
{"title":"Main Page","unwatched":"","message":"<p>The page \"<strong class=\"selflink\">Main Page</strong>\" has been removed from <a href=\"/wiki/Special:Watchlist\" title=Special:Watchlist>your watchlist</a>.\n</p>"}
]
}
```
Are clients expected to do something with this property?
If a client is using continuation, should they do something other than passing back the given `continue` value until exhausted?
I assume it doesn't let them influence the continuation, but one use case I can imagine is the user not being interested in part of the result and they might top following the continuation chain after `batchcomplete` and thus not bother starting the generator. Is that the (only) intended use?