scylla-gdb: document scylla_shard
scylla-gdb: document scylla_shard

The command is quite straightforward, but it didn't offer
any documentation when calling `help scylla shard`, so it's
hereby added. As a small bonus, a more comprehensive message
is printed when the argument is not an integer.
Message-Id: <dd790efac7df467a88fb22d281200234c0836b28@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla