LU-12837 doc: add lfs-changelog* manpages

LU-12837 doc: add lfs-changelog* manpages

This patch moves the documentation for "lfs changelog" and "lfs
changelog_clear" utilities from "lfs.1" to the following manpages:
- lfs-changelog.1
- lfs-changelog_clear.1

Signed-off-by: Etienne AUJAMES <eaujames@ddn.com>
Test-Parameters: trivial
Change-Id: I6db2e687e506a6116fe4755358a9abbd5509c3bb
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49209
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>