Clarify C++11 styleguide wording around std::to_string.

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

Index: styleguide/c++/c++11.html
diff --git a/styleguide/c++/c++11.html b/styleguide/c++/c++11.html
index 123ecd8651ef92be7ba36289e6a8036ed026f0f4..25c6f5fd6b2f451dd5e5d05330716ba609d216bf 100644
--- a/styleguide/c++/c++11.html
+++ b/styleguide/c++/c++11.html
@@ -866,9 +866,9 @@ work in all our compilers yet.</p>
 </tr>
 
 <tr>
-<td>String to Number Functions</td>
+<td>String-Number Conversion Functions</td>

[dcheng, 2016/07/25 04:19:59]

In the standard, these are called "Numeric conversions" (see 21.5
[string.conversions]), but that seemed a bit too vague. I'm not sure if this
still implies a unidirectional conversion too strongly.

 <td><code>std::stoi()</code>, <code>std::stol()</code>, <code>std::stoul()</code>, <code>std::stoll</code>, <code>std::stoull()</code>, <code>std::stof()</code>, <code>std::stod()</code>, <code>std::stold()</code>, <code>std::to_string()</code></td>
-<td>Converts strings to numbers</td>
+<td>Converts strings to/from numbers</td>

[dcheng, 2016/07/25 04:19:59]

I tried to imply that this covers conversions in both directions.

 <td><a href="http://en.cppreference.com/w/cpp/string/basic_string/stol">std::stoi, std::stol, std::stoll</a>, <a href="http://en.cppreference.com/w/cpp/string/basic_string/stoul">std::stoul, std::stoull</a>, <a href="http://en.cppreference.com/w/cpp/string/basic_string/stof">std::stof, std::stod, std::stold</a></td>

[chili, 2016/07/25 06:00:14]

the std::to_string link is missing here as well?

http://en.cppreference.com/w/cpp/string/basic_string/to_string

[dcheng, 2016/07/25 06:41:22]

Done.

 <td></td>
 </tr>