feature #6186 [Console] Add FormatterHelper::truncate docs (mheki)

This PR was squashed before being merged into the master branch (closes #6186).

Discussion
----------

[Console] Add FormatterHelper::truncate docs

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#16652)
| Applies to    | 3.1
| Fixed tickets | -

Commits
-------

60548b1 [Console] Add FormatterHelper::truncate docs

Author: xabbuh

components/console/helpers/formatterhelper.rst

@@ -63,3 +63,55 @@ messages and 2 spaces on the left and right).
 The exact "style" you use in the block is up to you. In this case, you're using
 the pre-defined ``error`` style, but there are other styles, or you can create
 your own. See :ref:`components-console-coloring`.
+
+Print Truncated Messages
+------------------------
+
+.. versionadded:: 3.1
+    The ``truncate`` method was introduced in Symfony 3.1.
+
+Sometimes you want to print a message truncated to an explicit character length.
+This is possible with the
+:method:`Symfony\\Component\\Console\\Helper\\FormatterHelper::truncate` method.
+
+If you would like to truncate a very long message, for example, to 7 characters,
+you can write::
+
+    $message = "This is a very long message, which should be truncated";
+    $truncatedMessage = $formatter->truncate($message, 7);
+    $output->writeln($truncatedMessage);
+
+And the output will be::
+
+    This is...
+
+The message is truncated to the given length, then the suffix is appended to end
+of that string.
+
+Negative String Length
+~~~~~~~~~~~~~~~~~~~~~~
+
+If the length is negative, the number of characters to truncate is counted
+from the end of the string::
+
+    $truncatedMessage = $formatter->truncate($message, -5);
+
+This will result in::
+
+    This is a very long message, which should be trun...
+
+Custom Suffix
+~~~~~~~~~~~~~
+
+By default, the ``...`` suffix is used. If you wish to use a different suffix,
+simply pass it as the third argument to the method.
+The suffix is always appended, unless truncate length is longer than a message
+and a suffix length.
+If you don't want to use suffix at all, just pass an empty string::
+
+    $truncatedMessage = $formatter->truncate($message, 7, '!!'); // result: This is!!
+    $truncatedMessage = $formatter->truncate($message, 7, '');   // result: This is
+    $truncatedMessage = $formatter->truncate('test', 10));
+    /* result: test
+       because length of the "test..." string is shorter than 10 */
+