Add statement for each additional instructor on add course form.

Add an "Enter information for additional instructor number X."
statement for each additional instructor to clarify what inputs
go with what instructor, and to break the inputs apart a little
better.

templates/ContentGenerator/CourseAdmin/add_course_form.html.ep

@@ -78,8 +78,11 @@
 				. 'The user ID may contain only numbers, letters, hyphens, periods (dots), commas, and underscores.'
 		) =%>
 	</div>
-	<div class="mb-1">
+	<div class="mb-2">
 	% for (1 .. $number_of_additional_users) {
+		<div class="mb-2">
+			<%= maketext('Enter information for additional instructor number [_1].', $_) %>
+		</div>
 		<div class="row mb-1">
 			<div class="col-lg-4 col-md-5 col-sm-6">
 				<div class="form-floating mb-1">