clarify SIS batch mode documentation

closes CNVS-34488

test plan: read api SIS batch mode section, make sure it makes sense

Change-Id: Ib716675c1e9542ac96415ec1c5f82b449df749f5
Reviewed-on: https://gerrit.instructure.com/100342
Tested-by: Jenkins
Reviewed-by: Tyler Pickett <tpickett@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>

doc/api/sis_csv.md

@@ -33,8 +33,10 @@ must be specified when uploading the SIS import. Use this option with caution, a
 large data sets without any prompting on the individual records. Currently, this affects courses,
 sections and enrollments.
 
-This option will only affect data created via previous SIS imports. Manually created courses, for
-example, won't be deleted even if they don't appear in the new SIS import.
+This option will only affect data that has been involved in a previous SIS job
+-- either created by a previous import, or referenced by a SIS job after a SIS
+ID was manually added. Manually created courses with no SIS ID, for example,
+won't be deleted even if they don't appear in the new SIS import.
 
 Diffing Mode
 ------------