Adds missing content to the 'Handling delayed data' page (#1497)

### 📸
[Preview](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/1497/explore-analyze/machine-learning/anomaly-detection/ml-delayed-data-detection)

### Description

Due to an error during the migration, the _Handling delayed data_ page
was missing some of its content. This PR restores the missing content.

### Related issue: #1460

Author: kosabogi

explore-analyze/images/ml-annotations.png

@@ -17,3 +17,67 @@ When you create a {{dfeed}}, you can specify a [`query_delay`](https://www.elast
 ::::{important}
 If you get an error that says `Datafeed missed XXXX documents due to ingest latency`, consider increasing the value of query_delay. If it doesn’t help, investigate the ingest latency and its cause. You can do this by comparing event and ingest timestamps. High latency is often caused by bursts of ingested documents, misconfiguration of the ingest pipeline, or misalignment of system clocks.
 ::::
+
+## Why worry about delayed data?
+
+If data are delayed randomly (and consequently are missing from analysis), the
+results of certain types of functions are not really affected. In these
+situations, it all comes out okay in the end as the delayed data is distributed
+randomly. An example would be a `mean` metric for a field in a large collection
+of data. In this case, checking for delayed data may not provide much benefit.
+If data are consistently delayed, however, {{anomaly-jobs}} with a `low_count`
+function may provide false positives. In this situation, it would be useful to
+see if data comes in after an anomaly is recorded so that you can determine a
+next course of action.
+
+## How do we detect delayed data?
+
+In addition to the `query_delay` field, there is a delayed data check config,
+which enables you to configure the datafeed to look in the past for delayed data.
+Every 15 minutes or every `check_window`, whichever is smaller, the datafeed
+triggers a document search over the configured indices. This search looks over a
+time span with a length of `check_window` ending with the latest finalized bucket.
+That time span is partitioned into buckets, whose length equals the bucket span
+of the associated {{anomaly-job}}. The `doc_count` of those buckets are then
+compared with the job's finalized analysis buckets to see whether any data has
+arrived since the analysis. If there is indeed missing data due to their ingest
+delay, the end user is notified. For example, you can see annotations in {{kib}}
+for the periods where these delays occur:
+
+:::{image} /explore-analyze/images/ml-annotations.png
+:alt: Delayed data annotations in the Single Metric Viewer
+:screenshot:
+:::
+
+::::{important}
+The delayed data check will not work correctly in the following cases:
+
+* if the {{dfeed}} uses aggregations that filter data,
+* if the {{dfeed}} uses aggregations and the job's `analysis_config` does not have
+its `summary_count_field_name` set to `doc_count`,
+* if the {{dfeed}} is _not_ using aggregations and `summary_count_field_name` is
+set to any value.
+
+If the datafeed is using aggregations, set the job's `summary_count_field_name`
+to `doc_count`. If `summary_count_field_name` is set to any value other than
+`doc_count`, the delayed data check for the datafeed must be disabled.
+::::
+
+There is another tool for visualizing the delayed data on the *Annotations* tab
+in the {{anomaly-detect}} job management page:
+
+:::{image} /explore-analyze/images/ml-datafeed-chart.png
+:alt: Delayed data in the {{dfeed}} chart
+:screenshot:
+:::
+
+## What to do about delayed data?
+
+The most common course of action is to simply to do nothing. For many functions
+and situations, ignoring the data is acceptable. However, if the amount of
+delayed data is too great or the situation calls for it, the next course of
+action to consider is to increase the `query_delay` of the datafeed. This
+increased delay allows more time for data to be indexed. If you have real-time
+constraints, however, an increased delay might not be desirable. In which case,
+you would have to [tune for better indexing speed](/deploy-manage/production-guidance/optimize-performance/indexing-speed.md). 
+