btrfs-progs: correct manpage option description for scrub

btrfs-progs: correct manpage option description for scrub

The -f option of scrub means to
    "force starting new scrub even if a scrub is already running"
*not*
    "force to check whether scrub has started or resumed in userspace"
as described originally.

So replace the orignal description in the manpage and code.

Also, add description of the potential failure as follows
    "If a scrub is already running, it fails."

Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Cc: David Sterba <dsterba@suse.cz>
Signed-off-by: David Sterba <dsterba@suse.cz>