should tell user we support time in string format and numerical format in command help.

[zhoucheng361]

What would you like to be added:

root@bench-01:~/juicefs# ./juicefs mount --help
--cache-expire value cached blocks not accessed for longer than this option will be automatically evicted (0 means never) (default: "0")
user don't know the unit of value

--cache-scan-interval value interval (in seconds) to scan cache-dir to rebuild in-memory index (default: "3600")
3600 should be 1h

we should tell the user we support time in string format and numerical format in command help.

Why is this needed: