use different markup for option descriptions

[marisademeglio]

replace the span with something block-level, because the contents may not just be inline elements, e.g. it could contain all this

<p>Metadata to be included in the EPUB.</p>
<p>If specified, the document must be a single
<a href="https://www.w3.org/publishing/epub3/epub-packages.html#sec-metadata-elem"><code>metadata</code></a> or
<a href="https://www.w3.org/publishing/epub3/epub-packages.html#sec-package-elem"><code>package</code></a> element. A
<a href="https://www.w3.org/publishing/epub3/epub-packages.html#sec-prefix-attr"><code>prefix</code></a> attribute is
allowed on the root element. The metadata will be injected in the EPUB's package document, possibly
overwriting existing metadata. This works as follows:</p>
<ul>
<li>All (valid) fields in the provided metadata document end up in the output EPUB. More than one
field with the same property is allowed. <code>meta</code> elements with a <code>refines</code> attribute must refine
elements within the metadata document itself. Elements that refine elements in the EPUB's package
document will be dropped.</li>
<li>Any metadata fields in the input EPUB that have matching fields (same property in case of <code>meta</code>
fields, same element name in case of <code>dc:*</code> fields) in the provided metadata document are omitted,
together with any <code>meta</code> elements that refine them.</li>
<li>Metadata fields in the input that do not have any matching fields in the provided metadata
document are preserved in the output.</li>
</ul>
<p>There are a number of fields that result in addional changes in the EPUB (apart from an updated
<code>metadata</code> section in the package document):</p>
<ul>
<li>If the provided metadata document contains one or more
<a href="https://www.w3.org/publishing/epub3/epub-packages.html#sec-opf-dcidentifier"><code>dc:identifier</code></a>
fields, the first one without a <code>refines</code> attribute will be used to update the
<a href="https://www.w3.org/publishing/epub3/epub-packages.html#attrdef-package-unique-identifier"><code>unique-identifier</code></a>
attribute on the package document. The <code>dc:identifier</code> metadata in the content documents can also be
aligned with it. This behavior can be enabled or disabled with the "Update &lt;meta
name='dc:identifier'&gt; elements based on EPUB metadata" option.</li>
<li>If the provided metadata document contains one or more
<a href="https://www.w3.org/publishing/epub3/epub-packages.html#sec-opf-dctitle"><code>dc:title</code></a> fields, the
first one can also be used as the <code>title</code> element in the content documents. This behavior can be
enabled or disabled with the "Update &lt;title&gt; elements based on EPUB metadata" option.</li>
<li>If the provided metadata document contains exactly one
<a href="https://www.w3.org/publishing/epub3/epub-packages.html#sec-opf-dclanguage"><code>dc:language</code></a> field,
it can be used to update the <code>xml:lang</code> and <code>lang</code> attributes of the content documents. This
behavior can be enabled or disabled with the "Update 'lang' attributes based on metadata" option.</li>
</ul>
<p>Some fields are ignored:</p>
<ul>
<li>The <a href="https://www.w3.org/publishing/epub3/epub-packages.html#last-modified-date"><code>dcterms:modified</code></a>
field gets updated whenever Pipeline produces an EPUB. As a consequence, any <code>dcterms:modified</code>
fields in the provided metadata document are ignored.</li>
</ul>