Improve guidance for choosing kernel template parameters of scan based on memory requirements

[adamfidel]

From https://github.com/oneapi-src/oneDPL/pull/1320#pullrequestreview-2018418038:

Memory footprint in SLM of joint algorithms, and how to communicate memory footprint requirements to the end user to provide them the proper guidance for selecting kernel params. I think this remains somewhat unresolved (as the sycl feature is a black box), but I think we can make a GH issue, and improve on this in the future (if we feel unsatisfied with our guidance in the docs with the current code).