docs: clarify that parent_account_id is a required column

for an account csv sis import, the file must include the
parent_account_id column even if all the values in it are blank, so that
we can differentiate the file from a group import.

fixes CNVS-16875

test plan:
- check sis csv docs

Change-Id: I5c897a249886e8234a530ce9ea8e3b3dec88c093
Reviewed-on: https://gerrit.instructure.com/73205
Tested-by: Jenkins
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>

doc/api/sis_csv.md

@@ -229,8 +229,10 @@ interface, this is called the SIS ID.</td>
 <tr>
 <td>parent_account_id</td>
 <td>text</td>
-<td>The account identifier of the parent account.
-If this is blank the parent account will be the root account.</td>
+<td><b>Required column</b>. The account identifier of the parent account.
+If this is blank the parent account will be the root account. Note that even if
+all values are blank, the column must be included to differentiate the file
+from a group import.</td>
 </tr>
 <tr>
 <td>name</td>