[css-transitions] Clarify that the example related to forward and reverse transitions only works when there are two states.

Fixes https://lists.w3.org/Archives/Public/www-style/2013Feb/0570.html

Author: dbaron

css-transitions/Overview.bs

@@ -941,7 +941,9 @@ url: https://html.spec.whatwg.org/multipage/infrastructure.html#concept-event-di
           </p>
           <p>This provides a way for authors to specify different values
           of the 'transition-*' properties for the &ldquo;forward&rdquo;
-          and &ldquo;reverse&rdquo; transitions (but see <a
+          and &ldquo;reverse&rdquo; transitions,
+          when the transitions are between two states
+          (but see <a
           href="#reversing">below</a> for special reversing behavior when
           an <em>incomplete</em> transition is interrupted).  Authors can
           specify the value of 'transition-duration',

css-transitions/Overview.html

@@ -1447,7 +1447,9 @@ <h2 class="heading settled" data-level="3" id="starting"><span class="secno">3.
 
      <p>This provides a way for authors to specify different values
           of the <a class="property" data-link-type="propdesc">transition-*</a> properties for the “forward”
-          and “reverse” transitions (but see <a href="#reversing">below</a> for special reversing behavior when
+          and “reverse” transitions,
+          when the transitions are between two states
+          (but see <a href="#reversing">below</a> for special reversing behavior when
           an <em>incomplete</em> transition is interrupted).  Authors can
           specify the value of <a class="property" data-link-type="propdesc" href="#propdef-transition-duration">transition-duration</a>,
           <a class="property" data-link-type="propdesc" href="#propdef-transition-timing-function">transition-timing-function</a>, or <a class="property" data-link-type="propdesc" href="#propdef-transition-delay">transition-delay</a> in the same