Update the config doc

[rkuchan]

At least:

• In the [http] section:

  unix-socket-enabled = true # enable http service over unix domain socket
  bind-socket = "/var/run/influxdb.sock"

(influxdata/influxdb#7135)

• In the [data] section: max-values-per-tag = 100000. Can be disabled with 0.
  Existing measurements with tags that exceed this limit will continue to load, but writes that would cause the tags cardinality to increase will be dropped and a partial write error will be returned to the caller. This limit can be used to prevent high cardinality tag values from being written to a measurement.

• defaults for cache-max-memory-size from 524288000 to 1048576000, cache-snapshot-write-cold-duration from 1h to 10m, and compact-full-write-cold-duration from 24h to 4h

• do general run-through