Array: Insufficient usage documentation

[adbridge]

The documenation explaining how to use this utility is quite sparse and needs to be extended.

[ciarmcom]

ARM Internal Ref: IOTSFW-2003

[adbridge]

A particular point of note is that the interface for PoolAllocator::align_up() states that the alignment parameter must be a power of 2. This is not stated in the alignment parameter for the array init function, so should be added. If we decide to remove the alignment property (as per the optimisation issue) then this will not need updating .