Include pipeline pause in limitations update for CHNG

Author: krivard

docs/api/covidcast-signals/chng.md

@@ -17,6 +17,8 @@ grand_parent: COVIDcast Epidata API
 
 ## Overview
 
+**Notice: This data source was paused on 2021-10-04 so that we can resolve some problems with the data pipeline. [Additional details on this pause are available below](#pipeline-pause).**
+
 This data source is based on Change Healthcare claims data that has been
 de-identified in accordance with HIPAA privacy regulations. Change Healthcare is
 a healthcare technology company that aggregates data from many healthcare providers.
@@ -189,6 +191,20 @@ Indicator values for as-of dates before 2021-02-21 are merely estimates, as
 these indicators were not yet available in real time. Backfill behavior of
 these estimates is erratic and not indicative of current backfill behavior.
 
+### Pipeline pause
+
+Starting on October 4, 2021, a problem with the `chng` pipeline began causing it
+to mark some days of data as deleted in their most recent version. These
+spurious deletions affected all regions and `chng` signals from July 31 to
+August 3, 2021, and the affected date range would continue to grow by one day
+each day if we allowed the pipeline to continue running.
+
+On October 8, 2021, we paused the `chng` pipeline, and it will remain inactive
+until we can identify and implement a fix. In the meantime, the versions with
+the deletion markings have been removed, so that default (latest) queries and
+queries with as-of set to 20211004 or later will return the
+next-most-recently-updated value for these dates.
+
 ## Qualifying Conditions
 
 We receive data on the following six categories of counts: