IVGCVSW-6216 PCA: Documentation Updates
IVGCVSW-6216 PCA: Documentation Updates

 * Added the UseCustomMemoryAllocator interface to the Backend Developer Guide
 * Added the GetCapabilities interface to the Backend Developer Guide

Signed-off-by: David Monahan <3b586dba7987ebb7bd7d41c9c9a617a1180f81c0@arm.com>
Change-Id: I26e96ec1b3fe74b52c770269f3f79e688d196667

ARM-software/armnn,ARM-software/armnn,ARM-software/armnn