Documentation: kernel-api: drop "Data Types" section

In the kernel-api chapter, the section for Data Types only
contains "Doubly Linked Lists" and all of the function interfaces
for list management.  There are no other data types in this section,
so collapse this section into "List Management Functions".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>