Improve DSL reference documentation to work by default

[cmacknz]

This is a follow up from #36649

The default DSL pattern is currently defined as follows (using Filebeat as an example):

beats/filebeat/filebeat.reference.yml

Lines 2445 to 2447 in 1d70a83

	# Set the lifecycle policy name or pattern. For DSL, this name must match the data stream that the lifecycle is for.
	# The default data stream pattern is filebeat-%{[agent.version]}"
	#setup.dsl.data_stream_pattern: "filebeat-%{[agent.version]}"

This is correct in that the index pattern to match the version exactly should be filebeat-8.11.0, but the current example of setup.dsl.data_stream_pattern: "filebeat-%{[agent.version]}" does not work by default because the placeholder %{[agent.version]} wont' match anything.

We should ideally template in the actual stack version, which for docs purposes is defined in

beats/libbeat/docs/version.asciidoc

Line 1 in 1d70a83

:stack-version: 8.11.0

or

beats/libbeat/version/version.go

Line 21 in 1d70a83

const defaultBeatVersion = "8.11.0"

in Go.

If that cannot be done, we should default to setup.dsl.data_stream_pattern: "filebeat-*" which is too broad but will at least work by default for a user trying Beats for the first time.

As an additional documentation nit, the end of the DSL section in the reference configuration needs a newline at the end to be consistent with the other sections:

beats/filebeat/filebeat.reference.yml

Lines 2460 to 2464 in 1d70a83

	#setup.dsl.overwrite: false
	# =================================== Kibana ===================================
	# Starting with Beats version 6.0.0, the dashboards are loaded via the Kibana API.
	# This requires a Kibana endpoint configuration.

[fearful-symmetry]

So, technically the filebeat-%{[agent.version]} string is the default, as that's what the config will fall back on if you don't configure anything, and the built-in fmtstr library in libbeat will just resolve that to filebeat-8.11. My instinct says that the commented-out config should be the same as the actual default, and if anything we should either explain how the template works, or just change the documentation. Of course, setting the documentation to filebeat-8.11 would technically produce the same config as filebeat-%{[agent.version]}.

[cmacknz]

When I wrote this I did not even know that filebeat-%{[agent.version]} would be resolved by the Beat, I forgot this was the fmtstr syntax.

If it works as expected I'm fine keeping it if we explain this in the configuration.