mm: memblock: update comments and kernel-doc

* Remove comments mentioning bootmem
* Extend "DOC: memblock overview"
* Add kernel-doc comments for several more functions

[akpm@linux-foundation.org: fix copy-n-paste error]
Link: http://lkml.kernel.org/r/1549626347-25461-1-git-send-email-rppt@linux.ibm.com


Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>