Misc. documentation updates in kernel
Some things I noticed while reading code.
Mostly this fixes out-of-date comments, but I also added a little bit about how fobjs in struct kmalloc_slab works.
I am fairly confident in these changes, but someone might want to take take a careful look nonetheless.
I removed the comment saying the lock structure can't be the first element struct vm_map because I see that it actually became the first element since that comment was written. I hope I got that right.