Fix usage instructions about sstabledump -k and -x options

The options -k and -x of sstabledump admit multiple arguments, so users can include or exclude multiple partitions. The intended usage is, for example:

$ sstablepartitions <sstable_path> -k 1
$ sstablepartitions <sstable_path> -k 1 2 3

However, the following command will fail:

$ sstablepartitions -k 1 <sstable_path>
You must supply exactly one sstable
usage: sstabledump <sstable file path> <options>

Dump contents of given SSTable to standard output in JSON format.
 -d         CQL row per line internal representation
 -e         enumerate partition keys only
 -k <arg>   Partition key
 -t         Print raw timestamps instead of iso8601 date strings
 -x <arg>   Excluded partition key

This command fails because the sstable path is considered a fourth partition key, and the mandatory argument for the sstable path is missed. While I think this behaviour is correct, it can be a bit confusing for users, especially when the information about usage describes both -k and -x as single-argument.

I think that at least we should fix the description of the options to indicate that there could be multiple included/excluded keys, and probably improve the message about the missing sstable path when those options are used.

Alternatively we could modify the options to have a single argument and allow to repeat them, so we could accept things like:

$ sstablepartitions -k 1 <sstable_path>
$ sstablepartitions -k 1 -k 2 -k 3 <sstable_path>
$ sstablepartitions <sstable_path> -k 1 -k 2 -k 3

The main downside of the latter approach is that the change in the syntax of the command might cause compatibility issues. Also we would need to upgrade commons-cli to at least 1.2 due to CLI-137.