code review from Josh and Robert

*** 68,78 ****
  .Pp
  .Fn vmem_create
  and
  .Fn vmem_destroy
  create and destroy vmem arenas.  In order to differentiate between arenas used
! for adresses and arenas used for identifiers, the
  .Dv VMC_IDENTIFIER
  flag is passed to
  .Fn vmem_create .
  This prevents identifier exhaustion from being diagnosed as general memory
  failure.
--- 68,78 ----
  .Pp
  .Fn vmem_create
  and
  .Fn vmem_destroy
  create and destroy vmem arenas.  In order to differentiate between arenas used
! for addresses and arenas used for identifiers, the
  .Dv VMC_IDENTIFIER
  flag is passed to
  .Fn vmem_create .
  This prevents identifier exhaustion from being diagnosed as general memory
  failure.
*** 81,91 ****
  .Em spans ,
  or contiguous ranges of integers.  For example, the kernel heap consists of
  just one span:
  .Li "[kernelheap, ekernelheap)" .
  Spans can be added to an arena in two ways: explicitly, by
! .Fn vmem_add ,
  or implicitly, by importing, as described in
  .Sx Imported Memory
  below.
  .Ss Segments
  Spans are subdivided into
--- 81,91 ----
  .Em spans ,
  or contiguous ranges of integers.  For example, the kernel heap consists of
  just one span:
  .Li "[kernelheap, ekernelheap)" .
  Spans can be added to an arena in two ways: explicitly, by
! .Fn vmem_add ;
  or implicitly, by importing, as described in
  .Sx Imported Memory
  below.
  .Ss Segments
  Spans are subdivided into
*** 120,134 ****
  requests) is just a subset of
  .Sy heap_arena
  (the kernel heap) that provides caching for the most common slab sizes.  When
  .Sy kmem_va_arena
  runs out of virtual memory, it
! .Em imports more from the heap; we
! say that
  .Sy heap_arena
  is the
! .Em "vmem source" for
  .Sy kmem_va_arena.
  .Fn vmem_create
  allows you to specify any existing vmem arena as the source for your new
  arena.  Topologically, since every arena is a child of at most one source, the
  set of all arenas forms a collection of trees.
--- 120,135 ----
  requests) is just a subset of
  .Sy heap_arena
  (the kernel heap) that provides caching for the most common slab sizes.  When
  .Sy kmem_va_arena
  runs out of virtual memory, it
! .Em imports
! more from the heap; we say that
  .Sy heap_arena
  is the
! .Em "vmem source"
! for
  .Sy kmem_va_arena.
  .Fn vmem_create
  allows you to specify any existing vmem arena as the source for your new
  arena.  Topologically, since every arena is a child of at most one source, the
  set of all arenas forms a collection of trees.