Update the documentation to reflect new type-aware kmalloc-family as
suggested in commit 2932ba8d9c99 ("slab: Introduce kmalloc_obj() and family")

I have also thought about adding a few cases to checkpatch.pl, but this
will take me some time, and i don't know if i can do it.

 [v1] -> [v2]:
put RCU/* in a seperate patch [Patch 2/3]
Omit optional argument (GFP_KERNEL) as suggested by https://lwn.net/Articles/1062856/
deprecated.rst: change the argument gfp to optional [Patch 3/3]

Signed-off-by: Manuel Ebner <manuelebner@mailbox.org>