Explicitly document that NavigationRegion.targets cannot be empty.

pkg/analysis_server/doc/api.html

Index: pkg/analysis_server/doc/api.html
diff --git a/pkg/analysis_server/doc/api.html b/pkg/analysis_server/doc/api.html
index 901200fc2989ffdda15251f24ca82142da4fd3d7..feaa424ab018d6ba2cf49204a93ddb63dda61da7 100644
--- a/pkg/analysis_server/doc/api.html
+++ b/pkg/analysis_server/doc/api.html
@@ -3142,7 +3142,7 @@ dt.typeDefinition {
             <p>
               The indexes of the targets (in the enclosing navigation response)
               to which the given region is bound. By opening the target, clients
-              can implement one form of navigation.
+              can implement one form of navigation. This list cannot be empty.
             </p>
           </dd></dl></dd><dt class="typeDefinition"><a name="type_NavigationTarget">NavigationTarget: object</a></dt><dd>
         <p>