scylla-gdb.py: fix existing documentation
scylla-gdb.py: fix existing documentation

Some commands are documented, but not in the python way. Refactor these
commands so they use the standard python way for self documenting. In
addition to being more "python", this makes these documentation strings
discoverable by GDB so they appear in the `help scylla` output.

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