Change field labels for docs comments form

Clearly indicate what's subject, what's body, and that it's sending an
email.

pgweb/docs/views.py

@@ -162,4 +162,5 @@ def commentform(request, itemid, version, filename):
 		'form_intro': template_to_string('docs/docsbug.html', {
 			'user': request.user,
 		}),
+		'savebutton': 'Send email',
 	})