Add explanatory text to admin twofactor settings

Ref #12249
Ref https://github.com/nextcloud/documentation/pull/937

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

settings/src/components/AdminTwoFactor.vue

@@ -49,6 +49,12 @@
 						 :close-on-select="false">
 			</Multiselect>
 		</p>
+		<p>
+			<em>
+				<!-- this text is also found in the documentation. update it there as well if it ever changes -->
+				{{ t('settings', 'When groups are selected/excluded, they use the following logic to determine if a user has 2FA enforced: If no groups are selected, 2FA is enabled for everyone except members of the excluded groups. If groups are selected, 2FA is enabled for all members of these. If a user is both in a selected and excluded group, the selected takes precedence and 2FA is enforced.') }}
+			</em>
+		</p>
 		<p>
 			<button class="button primary"
 					v-on:click="saveChanges"