Dl4J memory page: review + improve docs/examples

https://deeplearning4j.org/memory#configuring-memory-limits

Mainly around maxphysicalbytes: should be set to heap + maxbytes

More examples (that specify the exact limits for all types of memory, and behaviour for GPUs) would be good.

## Due Date
*To be completed by:* YYYY-MM-DD

## Description
*Write a short description of what needs to be done.*

## Assignees
*Please ensure you have assigned at least one person to this issue. Include any authors and reviewers required.*