LU-930 docs: add lfs-rm_entry.8 man page

LU-930 docs: add lfs-rm_entry.8 man page

Add man page for "lfs rm_entry" and alias "lfs rmentry".

Test-Parameters: trivial
Fixes: 2ad263c602 ("LU-1187 utils: add lfs setdirstripe/getdirstripe")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I783f23ec8fd0c75c69bcc78c180a07e54dd0c8a1
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49064
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Qian Yingjin <qian@ddn.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>