Add documentation for comments disabling/enabling formatting in code (// clang-format off).

[curdeius]

Summary

Adds documentation for `// clang-format off and // clang-format on` coments.

[curdeius]

Ping.

[djasper]

Looks good. Sorry that this slipped by.

[curdeius]

Please land it for me (no write access).

[djasper]

Done in r219204.

docs/ClangFormatStyleOptions.rst

 When specifying configuration in the ``-style=`` option, the same configuration
 is applied for all input files. The format of the configuration is:
 
 .. code-block:: console
 
   -style='{key1: value1, key2: value2, ...}'
 
 
+Disabling Formatting on a Piece of Code
+=======================================
+
+Clang-format understands also special comments that switch formatting in a
+delimited range. The code between a comment ``// clang-format off`` or
+``/* clang-format off */`` up to a comment ``// clang-format on`` or
+``/* clang-format on */`` will not be formatted. The comments themselves
+will be formatted (aligned) normally.
+
+.. code-block:: c++
+
+  int formatted_code;
+  // clang-format off
+      void    unformatted_code  ;
+  // clang-format on
+  void formatted_code_again;
+
+
 Configuring Style in Code
 =========================
 
 When using ``clang::format::reformat(...)`` functions, the format is specified
 by supplying the `clang::format::FormatStyle
 <http://clang.llvm.org/doxygen/structclang_1_1format_1_1FormatStyle.html>`_
 structure.