docs(developer-guide): the delete-record column is client-owned

The pre-edit hook example re-added _____DELETE__THIS__RECORD_____="No" when
it re-read the table, mirroring the data step getdata.sas used to run. That
column is created by the editor itself (it is the Delete? column) and is
never part of the dataset, so the example no longer adds it - and a response
that carries the name is refused by the editor, which the "What the hook
does" bullet now states.

Also notes the reserved name on the MPE_TABLES page, and says in the user
guide that the Delete? column is created by Data Controller rather than
being part of the table.

docs/dcc-tables.md

@@ -16,6 +16,9 @@ Adding tables to the Data Controller is a matter of configuration, specifically
 
 ![screenshot](img/configtable.png)
 
+!!! note
+    `_____DELETE__THIS__RECORD_____` is a reserved name. The editor creates that column itself - it is the **Delete?** column - and never reads it from the table, so a target table must not contain a column of that name. The editor refuses to open a table that does, rather than let the two collide.
+
 ## MPE_TABLES Configuration Details
 
 Each table to be edited in the Data Controller is represented by one record in `DATACTRL.MPE_TABLES`. The fields should be populated as follows: