editorial: Readd text saying requests may be denied based on battery …

…level. (w3c#328)

Follow-up to commit 0080849 ('Cleanup priv-sec wording + recommend an
indicator (w3c#323)').

@David-Chadwick suggested adding this part of the original text back because
it is still relevant (see w3c#323).

index.html

@@ -773,6 +773,11 @@ <h2>
         of the user to make or receive calls and use network services,
         including the emergency call service.
       </p>
+      <p>
+        Implementations MAY ignore requests for screen wake lock if, for
+        example, the battery capacity is low, or the user has put their device
+        in a power-saving mode.
+      </p>
       <p>
         It is RECOMMENDED that a user agent provide some UI or indicator that
         allows the user to know when a screen wake lock is active. Providing