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

std::to_string() is technically a number to string conversion function,
so clarify the wording to make it unambiguous.

BUG=none

Review-Url: https://codereview.chromium.org/2181573002
Cr-Commit-Position: refs/heads/master@{#407740}

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

@@ -866,10 +866,10 @@ work in all our compilers yet.</p>
 </tr>
 
 <tr>
-<td>String to Number Functions</td>
+<td>String-Number Conversion Functions</td>
 <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><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>
+<td>Converts strings to/from numbers</td>
+<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>, <a href="http://en.cppreference.com/w/cpp/string/basic_string/to_string">std::to_string</a></td>
 <td></td>
 </tr>