Beyond Mark's explanation of how to edit the placeholder text, it's possible to make text stay in the field when you edit it, although i have no idea why you would want to as the text will be submitted into the post unless removed by the user. :P
You will need to edit a core file, and re-apply every time you update.
<?= @text('Enter some text') ?>"><?= @escape($post->text) ?>
Insert the text you want to stick inbetween ><
<?= @text('Enter some text') ?>">STAY HERE TEXT<?= @escape($post->text) ?>
Again, i'm not sure why you would want to do this unless i have misunderstood the request?