docs/alternator/alternator.md: improve "Running Alternator" section
docs/alternator/alternator.md: improve "Running Alternator" section

A user complained that the "Running Alternator" section was confusing.
It didn't say outright which two configurations are necessary and you
had to read a few paragraph to reach it, and it mixed the YAML names
of options and the command-line names, which are subtly different.

This patch tries to improve this.

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