Add a "manual since" for the TimeoutError introduced in 5.2

The commit 0f13f6ff introduced this for 5.2, but then "since" tag was ignored as
there is no such support available by qdoc. Still, it is useful to make the end
users aware of this manually.

Change-Id: I46a642ddb8443f5c87d47709fa52921d7fd637a3
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>