[APM] Make sure to document all APM options

[sorenlouv]

Unless otherwise stated, every config option in APM should be publicly documented. The options can be found here:

kibana/x-pack/plugins/apm/server/index.ts

Lines 72 to 106 in 4fe8605

	const mergedConfig = {
	/* eslint-disable @typescript-eslint/naming-convention */
	// TODO: Remove all apm_oss options by 8.0
	'apm_oss.transactionIndices': apmOssConfig.transactionIndices,
	'apm_oss.spanIndices': apmOssConfig.spanIndices,
	'apm_oss.errorIndices': apmOssConfig.errorIndices,
	'apm_oss.metricsIndices': apmOssConfig.metricsIndices,
	'apm_oss.sourcemapIndices': apmOssConfig.sourcemapIndices,
	'apm_oss.onboardingIndices': apmOssConfig.onboardingIndices,
	/* eslint-enable @typescript-eslint/naming-convention */
	'xpack.apm.serviceMapEnabled': apmConfig.serviceMapEnabled,
	'xpack.apm.serviceMapFingerprintBucketSize':
	apmConfig.serviceMapFingerprintBucketSize,
	'xpack.apm.serviceMapTraceIdBucketSize':
	apmConfig.serviceMapTraceIdBucketSize,
	'xpack.apm.serviceMapFingerprintGlobalBucketSize':
	apmConfig.serviceMapFingerprintGlobalBucketSize,
	'xpack.apm.serviceMapTraceIdGlobalBucketSize':
	apmConfig.serviceMapTraceIdGlobalBucketSize,
	'xpack.apm.serviceMapMaxTracesPerRequest':
	apmConfig.serviceMapMaxTracesPerRequest,
	'xpack.apm.ui.enabled': apmConfig.ui.enabled,
	'xpack.apm.maxServiceEnvironments': apmConfig.maxServiceEnvironments,
	'xpack.apm.maxServiceSelection': apmConfig.maxServiceSelection,
	'xpack.apm.ui.maxTraceItems': apmConfig.ui.maxTraceItems,
	'xpack.apm.ui.transactionGroupBucketSize':
	apmConfig.ui.transactionGroupBucketSize,
	'xpack.apm.autocreateApmIndexPattern': apmConfig.autocreateApmIndexPattern,
	'xpack.apm.telemetryCollectionEnabled':
	apmConfig.telemetryCollectionEnabled,
	'xpack.apm.searchAggregatedTransactions':
	apmConfig.searchAggregatedTransactions,
	'xpack.apm.metricsInterval': apmConfig.metricsInterval,
	'xpack.apm.agent.migrations.enabled': apmConfig.agent.migrations.enabled,
	};

And documentation is located here:
https://github.com/elastic/kibana/blob/4fe8605c7b50e4a94e99fc63f64f05870ad449eb/docs/settings/apm-settings.asciidoc

Add to Cloud allow list
Additionally, every option should be allowed on cloud unless there are specific reasons not to. Cloud allow list can be found here:

https://github.com/elastic/cloud/blob/242a0485fdd07149797bfc9a3c86a2726f710be7/scala-services/adminconsole/src/main/resources/settings/kibana/apm.yml

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[gbamparop]

Created a separate issue for the cloud allow list #114130