[docs] xpack.monitoring.history.duration doesn't accept month

[kiju98]

https://www.elastic.co/guide/en/elasticsearch/reference/current/monitoring-settings.html has

xpack.monitoring.history.duration
Sets the retention duration beyond which the indices created by a Monitoring exporter are automatically deleted. Defaults to 7d (7 days).

This setting has a minimum value of 1d (1 day) to ensure that something is being monitored, and it cannot be disabled.

I tested with 1m and 1M to set it to 1 month on Elasticsearch 6.6.0 but they produced the following error:

 “org.elasticsearch.bootstrap.StartupException: java.lang.IllegalArgumentException: failed to parse value [1m] for setting [xpack.monitoring.history.duration], must be >= [24h]”

Looks like both m and M are considered as minutes and I see it accepts h and s.

I think it would be better if it is added to document that xpack.monitoring.history.duration accepts only d for days, h for hours, m and M for minutes, s for seconds but not months.

[kiju98]

Pinging @elastic/es-docs

[jasontedor]

The way that we typically denote this in the docs is by linking to the TimeValue settings page which explains exactly what units are supported. That is what I think we should do here, and what I think we should do in general, to avoid repeating the same sentence over and over.

[elasticmachine]

Pinging @elastic/es-core-features

[jasontedor]

For anyone on the @elastic/es-core-features team that wants to pick this up, see follow-request-body.asciidoc for an example of how we denote this elsewhere in the docs.