MFC r327226, r327356

r327226:
Add a section describing how to tune ARM kernel options to use an MD_ROOT
filesystem larger than about 50-55 MiB.

The description of VM_KMEM_SIZE_SCALE is roughly as hand-wavy as my
understanding of the option, but at least mentioning that it's a factor
and giving an empirical datapoint that works will give folks some idea
of what to tweak if they have problems.

r327356:
Correct a mistake and reword a couple sentences to clarify that "the value"
refers to the scale value, not the kmem_arena size that results from scaling.

Suggested by: alc@