[sysmem] doc comment re. required_* fields

Add a couple short paragraphs clarifying how required_min_size and
required_max_size interact with min_size and max_size.

Change-Id: I62a68486ea21068b27ad4ea44d0902ea8d9a36b7
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1387121
API-Review: Emircan Uysaler <emircan@google.com>
Reviewed-by: Josh Gargus <jjosh@google.com>
Reviewed-by: Emircan Uysaler <emircan@google.com>
Commit-Queue: Dustin Green <dustingreen@google.com>
1 file changed