nilfs2: improve kernel-doc comments for b-tree node helpers

nilfs2: improve kernel-doc comments for b-tree node helpers

Revise kernel-doc comments for helper functions related to changing the
search key for b-tree node blocks, and eliminate the following warnings
output by the kernel-doc script:

 fs/nilfs2/btnode.c:175: warning: Function parameter or struct member 'btnc'
  not described in 'nilfs_btnode_prepare_change_key'
 fs/nilfs2/btnode.c:175: warning: Function parameter or struct member 'ctxt'
  not described in 'nilfs_btnode_prepare_change_key'
 fs/nilfs2/btnode.c:238: warning: Function parameter or struct member 'btnc'
  not described in 'nilfs_btnode_commit_change_key'
 fs/nilfs2/btnode.c:238: warning: Function parameter or struct member 'ctxt'
  not described in 'nilfs_btnode_commit_change_key'
 fs/nilfs2/btnode.c:278: warning: Function parameter or struct member 'btnc'
  not described in 'nilfs_btnode_abort_change_key'
 fs/nilfs2/btnode.c:278: warning: Function parameter or struct member 'ctxt'
  not described in 'nilfs_btnode_abort_change_key'

Link: https://lkml.kernel.org/r/20240816074319.3253-4-konishi.ryusuke@gmail.com
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>