btrfs-progs: fix many typos in documents

btrfs-progs: fix many typos in documents

There are many trivial typos in Documentation/*.txt.
All of these use "exist status" to mean "exit status"
by mistake. I guess someone first made this mistake
and it has spread by copy-and-paste :-D

Signed-off-by: Naohiro Aota <naota@elisp.net>
Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>