Remove escaping of Fieldset descriptions.

At this moment when using a form field description
```XML
<fieldset description="COM_EXAMPLE_DESC" />
```
with language string
```INI
COM_EXAMPLE_DESC="This is <strong>important!</strong>"
```
The translation is being escaped in views:
```PHP
echo '<p class="tip">' . $this->escape(JText::_($fieldSet->description)) . '</p>';
```
preventing use HTML tags in the description:
This is &lt;strong&gt;important!&lt;/strong&gt;


However this is not a case for 
- the [spacer field](https://github.com/joomla/joomla-cms/blob/staging/libraries/joomla/form/fields/spacer.php#L83), which may be used as a workaround, 
- and other [field descriptions](https://github.com/joomla/joomla-cms/blob/staging/libraries/joomla/form/field.php#L717)

So I think HTML escaping should be removed for fieldset descriptions.
This allows to be more descriptive, add hyperlinks, images, etc.