notify-admin-642 document how the downloadable reports work

docs/downloadable_reports.md

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+# Downloadable reports for sent messages and how they work
+
+Downloadable reports related to sending messages are a little mysterious.  They can have a variable number of columns,
+some of which contain PII, and it is not immediately clear what drives what gets displayed.  This is an explanation.
+
+## Downloadable reports for one-off messages
+
+When a user sends an ad-hoc message by typing in a phone number and sending, the downloadable report is only going to
+show the bare minimum of columns.  There will be a column that shows the name of what template was used, but otherwise
+there will be nothing beyond the bare basics of the phone number and the time sent, etc.
+
+## Downloadable reports for jobs (uploaded csv files)
+
+When a user uploads a csv file -- creating a job -- the downloadable report becomes more complex and interesting.
+
+(Sample report)
+
+|Row number|Phone number|name|date|time|address|English|Spanish|Template|Type|Job|Status|Time|
+|----------|------------|----|----|----|-------|-------|-------|--------|----|---|------|----|
+|1|17169829002|Tim|10/16|2:00 PM|5678 Tom St.|no|yes|Appointment reminder - 1 week|sms|US Notify Demo CSV - Copy of Sheet1 (11).csv|Sending|2023-07-18 15:25:54| 
+
+
+In notifications_admin, in app.util.csv.py, there is a method called generate_notifications_csv().
+
+It is using the service_id and job_id to look up the csv file in s3. It is then merging the standard
+downloadable report (what we see in the one-off case mentioned above) with the custom csv data.
+
+This means that the PII displayed is mostly not stored in the database, but rather is stored in S3.
+
+The only PII stored in the database is the recipient's phone number, and that data is scrubbed.  If the
+sms message is delivered successfully, the phone number is scrubbed immediately.  If the sms message
+cannot be delivered, the PII will be scrubbed after seven days.