[web-animations-1] Add introductory text describing how properties are updated

Closes w3c#2359.

Author: birtles

web-animations-1/Overview.bs

@@ -386,6 +386,24 @@ and is only relevant to user agents that provide scripting support.
 
 </div>
 
+<h2 id="spec-conventions">Specification conventions</h2>
+
+This specification begins by describing abstract concepts such as [=animations=]
+and [=animation effects=] and properties that belong to them such as their
+[=playback rate=] or [=iteration duration=].
+In addition to these properties, there are often specific procedures for
+updating these properties such as the procedure to [=set the playback rate=] or
+the procedure to [=set the start time=] of an animation.
+
+Where this specification does not specifically link to a procedure, text
+that requires the user agent to update a property such as, &ldquo;make
+|animation|'s [=start time=] [=unresolved=]&rdquo;, should be understood to
+refer to updating the property directly <em>without</em> invoking any related
+procedure.
+
+Further documentation conventions that are not specific to this specification
+are described in [[#conventions]].
+
 <h2 id="web-animations-model-overview">Web Animations model overview</h2>
 
 <div class='informative-bg'><em>This section is non-normative</em>