styleguide: Fix/clarify the cbegin/cend rules are for container methods

Not for the global std::cbegin/cend, which are in C++14.

R=Nico, jbroman

Review URL: https://codereview.chromium.org/1669683002

Cr-Commit-Position: refs/heads/master@{#373604}

styleguide/c++/c++11.html

@@ -828,10 +828,10 @@ <h3 id="blacklist_stdlib_review"><a name="library-review"></a>C++11 Standard Lib
 
 <tr>
 <td>Constant Iterator Methods on Containers</td>
-<td><code>std::cbegin()</code> and <code>std::cend()</code></td>
+<td><code>std::vector::cbegin()</code> and <code>std::vector::cend()</code></td>
 <td>Enforces iteration methods that don't change container contents</td>
-<td>TODO: documentation link</td>
-<td></td>
+<td><a href="http://en.cppreference.com/w/cpp/container/vector/begin">std::vector::cbegin<a></td>
+<td>This applies to all containers, std::vector is just an example. <a href="https://groups.google.com/a/chromium.org/d/topic/cxx/cS83F_buqLM/discussion">Discussion thread</a></td>
 </tr>
 
 <tr>