Fix inaccurate code comment in Media.php

Problem/Motivation

In \Drupal\media\Entity\Media::prepareSave() we have:

        foreach ($translation->bundle->entity->getFieldMap() as $metadata_attribute_name => $entity_field_name) {
          // Only save value in entity field if empty. Do not overwrite existing
          // data.
          if ($translation->hasField($entity_field_name) && ($translation->get($entity_field_name)->isEmpty() || $translation->hasSourceFieldChanged())) {
            $translation->set($entity_field_name, $media_source->getMetadata($translation, $metadata_attribute_name));
          }
        }

That inline comment is incorrect. That was probably the original intention, but we might have changed the conditional and missed updating the comment.

Proposed resolution

A more accurate comment would be:

        foreach ($translation->bundle->entity->getFieldMap() as $metadata_attribute_name => $entity_field_name) {
          // Only save value in the entity if the field is empty or if the
          // source field changed.
          if ($translation->hasField($entity_field_name) && ($translation->get($entity_field_name)->isEmpty() || $translation->hasSourceFieldChanged())) {
            $translation->set($entity_field_name, $media_source->getMetadata($translation, $metadata_attribute_name));
          }
        }

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet