Add some more datasync docs

docs/data/sync/watchers.rst

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+
+==========
+ Watchers
+==========
+
+A datasync "watcher" is responsible for checking a given system to see
+if any records are in need of sync (e.g. have changed recently).
+
+If the given system has a SQL DB, and applicable tables include a
+"last modified" column, then the most common/simple way is for the
+watcher to simply query the table, looking for records which were
+modified since the last check.  There are a couple of downsides to
+this approach:
+
+* must query each table separately, to find all "changed" records
+* not possible to detect "deleted" records in this way
+
+Rattail itself uses a different approach though.  The Rattail DB
+includes a ``change`` table, which in normal circumstances is empty.
+If so configured, Rattail can insert records into this ``change``
+table whenever (almost) *any* data record is changed or deleted etc.
+Then a datasync watcher will look for records in the ``change`` table
+instead of having to query each applicable table separately.  The
+watcher also removes ``change`` records as they are processed.
+
+A similar effect can be achieved if the watched system has a SQL DB to
+which you can install (tables and) triggers.  In this case you can
+create a new table (e.g. ``datasync``) and then add triggers for the
+various tables you need to watch.  When any record is changed or
+deleted the trigger should add a record to your new ``datasync``
+table.  Then the actual watcher can check your ``datasync`` table, and
+remove any found after they are processed.
+
+Those are just the patterns used thus far; more are likely possible.