Mention sites/default/settings.php in example.settings.local.php

Problem/Motivation

In example.settings.local.php, the comment at the top states the following:

 * To activate this feature, copy and rename it such that its path plus
 * filename is 'sites/example.com/settings.local.php', where example.com
 * is the name of your site. Then, go to the bottom of
 * 'sites/example.com/settings.php' and uncomment the commented lines that
 * mention 'settings.local.php'.

In most cases, the correct location is sites/default/settings.local.php - it is only a minority of cases that have a site name in the path. We should help the 80% who have sites/default to get this step right, then give further instruction for the 20% who use a site name.

Proposed resolution

Something like this:

 * To activate this feature, copy and rename it such that its path plus
 * filename is 'sites/default/settings.local.php'. Then, go to the bottom of
 * 'sites/default/settings.php' and uncomment the commented lines that mention
 * 'settings.local.php'.
 *
 * If you are using a site name in the path, such as 'sites/example.com', copy
 * this file to 'sites/example.com/settings.local.php', and uncomment the lines
 * at the bottom of 'sites/example.com/settings.php'.