Clarify server and client notification-channel-discovery.

Issue #58

protocol.html

@@ -504,6 +504,8 @@ <h3 property="schema:name">Discovery</h3>
                   <section id="notification-channel-discovery" inlist="" rel="schema:hasPart" resource="#notification-channel-discovery">
                     <h4 property="schema:name">Notification Channel Discovery</h4>
                     <div datatype="rdf:HTML" property="schema:description">
+                      <p>Server supports discovery of Notification Channels through the use of Link headers. Clients can discover Notification Channel information available to a given resource or to a storage as follows:</p>
+
                       <p about="" id="server-link-resource-notification-channel" rel="spec:requirement" resource="#server-link-resource-notification-channel"><span property="spec:statement">When a server wants to enable applications to discover <a href="#notification-channel">Notification Channels</a> available to a given <a href="https://solidproject.org/TR/protocol#resource">resource</a>, the <span rel="spec:requirementSubject" resource="spec:Server">server</span> <span rel="spec:requirementLevel" resource="spec:MUST">MUST</span> advertise the associated resources describing the channel information by responding to an HTTP request including a <code>Link</code> header with the <code>rel</code> value of <code>describedby</code> [<cite><a class="bibref" href="#bib-powder-dr">POWDER-DR</a></cite>] and the Description Resource as link target [<cite><a class="bibref" href="#bib-rfc8288">RFC8288</a></cite>].</span></p>
 
                       <p about="" id="server-link-storage-notification-channel" rel="spec:requirement" resource="#server-link-storage-notification-channel"><span property="spec:statement">When a server wants to enable applications to discover <a href="#notification-channel">Notification Channels</a> available to a <a href="https://solidproject.org/TR/protocol#storage">storage</a> in which a given <a href="https://solidproject.org/TR/protocol#resource">resource</a> is in, the <span rel="spec:requirementSubject" resource="spec:Server">server</span> <span rel="spec:requirementLevel" resource="spec:MUST">MUST</span> advertise the associated resources describing the channel information by responding to an HTTP request including a <code>Link</code> header with the <code>rel</code> value of <code>http://www.w3.org/ns/solid#storageDescription</code> [<cite><a class="bibref" href="#bib-solid-protocol">SOLID-PROTOCOL</a></cite>] and the Description Resource as link target [<cite><a class="bibref" href="#bib-rfc8288">RFC8288</a></cite>].</span></p>