Explain distinction between logical containment and in-memory containment

[knoepfel]

Per the CDR reviewers' comment:

Emphasizing that data cells do not contain or own the associated data products, and that the data product lifetime is tied directly to the dataflow graph is an important detail that users may not have experienced in other HEP frameworks. This is necessary to meet the memory requirements for Phlex, but also means that users may not have full “control” over where their data is or when it exists.