headers/deps: mm: align MANITAINERS and Docs with new gfp.h structure

headers/deps: mm: align MANITAINERS and Docs with new gfp.h structure

After moving gfp types out of gfp.h, we have to align MAINTAINERS
and Docs, to avoid warnings like this:

>> include/linux/gfp.h:1: warning: 'Page mobility and placement hints' not found
>> include/linux/gfp.h:1: warning: 'Watermark modifiers' not found
>> include/linux/gfp.h:1: warning: 'Reclaim modifiers' not found
>> include/linux/gfp.h:1: warning: 'Useful GFP flag combinations' not found

Signed-off-by: Yury Norov <yury.norov@gmail.com>