Iptables input: document better the ignored rules behavior (influxdat…

…a#2482)

During issue influxdata#2215 it was highlighted that the current behavior where
rules without a comment are ignored is confusing for several users.

This commit improves the documentation and adds a NOTE to the sample
config to clarify the behavior for new users.

Telegraf 1.2.1

Telegraf 1.2.0

Telegraf 1.2 Release Candidate 3

Telegraf 1.2 Release Candidate 2

Telegraf 1.2.0 RC 1

telegraf development branch

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Telegraf 1.1.2

Telegraf 1.1.1

Telegraf 1.1.0