objectcache: Remove docs about RedisConnectionPool configurable size

There exists no option matching this description.

The class was created in Feb 2013 with Ie72536240e (03833b54e8),
which initially included a poolSize option that matches this
description.

This was removed the next month in March 2013,
with I6ea53378c9 (646051d36d).

While at it, fix file headers and ingroup doc annotations

Change-Id: I285122054c8848d0fa700aa323ff813d5f0c4690
(cherry picked from commit 91f7c7289363f982a28d3ba03ea3b19552db5360)