Add long descriptions to some plugins

Edited the short descriptions to answer the question
"What can I do with this extension" to help users decide
whether they need it.

In long descriptions, tell users what else they need to
be able to use the plugin.

Change-Id: Iefce7505b61fc77cf38cc915f1f7dbd25c9cb570
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/clangcodemodel/ClangCodeModel.json.in

@@ -13,7 +13,10 @@
                   "Alternatively, this plugin may be used under the terms of the GNU General Public License version 3 as published by the Free Software Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT included in the packaging of this plugin. Please review the following information to ensure the GNU General Public License requirements will be met: https://www.gnu.org/licenses/gpl-3.0.html."
     ],
     "Category" : "C++",
-    "Description" : "Clang Code Model plugin.",
+    "Description" : "Use Clang code model instead of the built-in code model",
+    "LongDescription" : [
+        "The code model is the part of an IDE that understands the programming language you are using to write your application. With it, Qt Creator can help you code faster and avoid errors."
+    ],
     "Url" : "https://www.qt.io",
     ${IDE_PLUGIN_DEPENDENCIES}
 }