LU-14109 doc: lnetctl manpage missing import and export options

LU-14109 doc: lnetctl manpage missing import and export options

Describe missing options in the lnetctl man page:
lnetctl import "--exec"
lnetctl export "--backup"

Test-Parameters: trivial

Lustre-change: https://review.whamcloud.com/40523
Lustre-commit: cc43eb49af8220c61dbd0b823d8bed7ffaeadf8b

Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Change-Id: I47e6c5e264dc8d5673f1229291be873996c02f55
Reviewed-on: https://review.whamcloud.com/40800
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Gian-Carlo DeFazio <defazio1@llnl.gov>
Reviewed-by: Oleg Drokin <green@whamcloud.com>