genalloc: Fix a set of docs build warnings

Commit 795ee306 ("lib/genalloc: introduce chunk owners") made a number
of changes to the genalloc API and implementation but did not update the
documentation to match, leading to these docs build warnings:

  ./lib/genalloc.c:1: warning: 'gen_pool_add_virt' not found
  ./lib/genalloc.c:1: warning: 'gen_pool_alloc' not found
  ./lib/genalloc.c:1: warning: 'gen_pool_free' not found
  ./lib/genalloc.c:1: warning: 'gen_pool_alloc_algo' not found

Fix these by updating the docs to match new function locations and names,
and by completing the update of one kerneldoc comment.

Fixes: 795ee306

 ("lib/genalloc: introduce chunk owners")
Acked-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>