Improve the description of node flags in man page
Improve the description of node flags in man page

[acf83a5274f31a43af6b914614365999469c1a31@google.com: slightly reworded the explanation for offline and
changed the commit message]
Signed-off-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>
Reviewed-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>

yiannist/ganeti,ganeti-github-testing/ganeti-test-1,bitemyapp/ganeti,apyrgio/ganeti,ganeti/ganeti,onponomarev/ganeti,bitemyapp/ganeti,grnet/snf-ganeti,leshchevds/ganeti,ganeti/ganeti,apyrgio/ganeti,andir/ganeti,leshchevds/ganeti,yiannist/ganeti,dimara/ganeti,mbakke/ganeti,leshchevds/ganeti,ganeti/ganeti,mbakke/ganeti,dimara/ganeti,yiannist/ganeti,andir/ganeti,grnet/snf-ganeti,apyrgio/snf-ganeti,onponomarev/ganeti,ganeti-github-testing/ganeti-test-1,andir/ganeti,apyrgio/snf-ganeti,mbakke/ganeti