docs(appsettings): each setting's description says what it does

The descriptions were tooltips, many only repeating the setting's name
("Cpu modes", "Debug for user quotas"). Now shown under each field, they
say what the setting changes: what a new VM or disk gets, how the
console starts, what a page shows. A migration writes them.

appsettings/tests/test_appsettings.py

@@ -99,7 +99,7 @@ class AppSettingsTestCase(TestCase):
         (field,) = page.xpath("//*[@name='LOGS_PER_PAGE']")
         self.assertEqual((field.get("type"), field.get("min"), field.get("max")), ("number", "1", "1000"))
         self.assertEqual(page.xpath(f"//label[@for='{field.get('id')}']/text()"), ["Logs per Page"])
-        self.assertEqual(page.xpath(f"//*[@id='{field.get('aria-describedby')}']/text()"), ["Pagination for logs"])
+        self.assertEqual(page.xpath(f"//*[@id='{field.get('aria-describedby')}']/text()"), ["Rows per page on the Logs page"])
         # a group is saved with its button, not on every change
         self.assertEqual(page.xpath("//*[@onchange]"), [])
 

appsettings/tests/test_migrations.py

@@ -87,3 +87,15 @@ class ChoicesFixTestCase(TestCase):
             "INSTANCE_MACHINE_DEFAULT_TYPE": ("q35", "q35,pc,virt"),
             "INSTANCE_CDROM_ADD": ("sata", "None,ide,sata,scsi"),
         })
+
+
+descriptions = importlib.import_module("appsettings.migrations.0021_setting_descriptions")
+
+
+class DescriptionsTestCase(TestCase):
+    def test_every_setting_says_what_it_does_within_its_column(self):
+        limit = AppSettings._meta.get_field("description").max_length
+        for key, text in descriptions.DESCRIPTIONS.items():
+            with self.subTest(key=key):
+                self.assertLessEqual(len(text), limit)
+                self.assertEqual(AppSettings.objects.get(key=key).description, text)