[css-transitions] Specify serialization for timing functions

See thread starting:
https://lists.w3.org/Archives/Public/www-style/2016Apr/0107.html

In particular, notice that Edge/Blink/Gecko all serialize 'steps(1)'
differently and at least Gecko and Blink differ with regards to how they
handle the optional 'end' / 'start' value.

css-transitions/Overview.bs

@@ -445,6 +445,26 @@ Introduction {#introduction}
         </dd>
       </dl>
 
+### Serializing a timing function ### {#serializing-a-timing-function}
+
+      Timing functions are serialized using the common serialization patterns
+      defined in [[CSSOM]] with the following additional requirements:
+
+      *   The keyword values ''ease'', ''linear'', ''ease-in'', ''ease-out'',
+          and ''ease-in-out'' are serialized as-is, that is, they are
+          <em>not</em> converted to the equivalent <a lt="cubic-bezier()"
+          function>cubic-bezier()</a> function before serializing.
+
+      *   Step timing functions, whether they are specified using the <a
+          lt="steps()" function>steps()</a> function or either of the
+          ''step-start'' or ''step-end'' keywords, are serialized as follows:
+
+          1.   If the point at which the value changes is ''end'', serialize
+               as <a lt="steps()" function>steps(&lt;integer&gt;)</a>.
+
+          2.   Otherwise, serialize as <a lt="steps()"
+               function>steps(&lt;integer&gt;, start)</a>.
+
 <span id="the-transition-delay-property-">The 'transition-delay' Property</span> {#transition-delay-property}
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