nilfs2: add kernel doc comments to persistent object allocator functions

The implementation of persistent object allocator (alloc.c) is poorly
documented.  This adds kernel doc style comments on that functions.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

diff --cc fs/nilfs2/alloc.c
Simple merge

diff --cc fs/nilfs2/alloc.h
Simple merge