The purpose here is to explain how memory pools work, what their
architecture is depending on the build options (4 possible combinations),
and how the various build options affect their behavior.
Two pool-specific macros that were previously documented in initcalls
were moved to pools.txt.
It's not always easy to figure that there are some docs on internal API
stuff, let's move them to their own directory. There's a diagram for
lists that could be placed there but instead would deserve a greppable
description for quick lookups, so it was not moved there.
