doc: various updates

[includes/cache/MessageCache.php]
- internal constants
- constructor
- a few missing @var

[includes/clientpool/RedisConnectionPool.php]
- group internal settings applying to the pool
- misc protected members updates

[includes/debug/Debug.php]
- missing parameter name in @var statements

Change-Id: I6ff0a68d659529d128f40f32b0fd1c1d39af952f
Merge "doc: various updates"