[Extension API Docs]Update tabs.highlight language

Updated description for clarity.

Bug: 740432
Change-Id: Ic7c8c7f96738d77c7b188e0b08d637f97b7bc31f
Reviewed-on: https://chromium-review.googlesource.com/957864Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Crystal Lambert <crystallambert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543817}

chrome/common/extensions/api/tabs.json

@@ -524,7 +524,7 @@
       {
         "name": "highlight",
         "type": "function",
-        "description": "Highlights the given tabs.",
+        "description": "Highlights the given tabs and focuses on the first of group. Will appear to do nothing if specified tab is currently active.",
         "parameters": [
           {
             "type": "object",