$wgHooks: add closure docs & admonition to register handlers early

This patch takes Tim's advice to amend the doc block for $wgHooks with an
admonition to register hook handlers early. (This was after it was determined
that late registration of a CanonicalNamespace handler was responsible for bug
45031). Since I was already here, I also documented the use of closures with
$wgHooks, fulfilling a @TODO.

Change-Id: Id16148dbfbcc89e0365860e078e089ae541ba08f
Merge "$wgHooks: add closure docs & admonition to register handlers early"