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

Lustre-change: https://review.whamcloud.com/49209
Lustre-commit: 82e7ad348c77e5c164aa3e3155c9eb91872369d5

Signed-off-by: Etienne AUJAMES <eaujames@ddn.com>
Signed-off-by: Xing Huang <hxing@ddn.com>
Test-Parameters: trivial
Change-Id: I6db2e687e506a6116fe4755358a9abbd5509c3bb
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53471
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>