Added explainer para below H1 (#371)

Added text to help user understand that when you edit a batch, the
changes will only apply to records completed in future. The changes to
the batch details are not applied retrospectively to records that
included the batch in question.

Author: Anna-Sutton

app/views/vaccines/edit-batch.html

@@ -12,7 +12,9 @@
   <div class="nhsuk-grid-row">
     <div class="nhsuk-grid-column-two-thirds">
 
-      <h1 class="nhsuks-heading-xl">Edit batch</h1>
+      <h1 class="nhsuks-heading-l">Edit batch</h1>
+
+      <p class="nhsuk-body-m">Your changes will only apply to vaccination records saved in future.</p>
 
       <form action="/vaccines/{{ vaccine.id }}/{{ batch.batchNumber }}/update" method="post" novalidate="true">