(bug 44244) Add number of watchers to API prop=info

Following up I1cb6ee22, which added this information to each page's
action=info, this adds inprop=watchers to query the number of people
watching the page. It is subject to the same limitations (user has
unwatchedpages or watchers >= $wgUnwatchedPageThreshold) as action=info.

Also, update doc for $wgUnwatchedPageThreshold to match reality.

Bug: 44244
Change-Id: Ideaac1d84bbe0349154ffe96ba54d74305e3da1d