[clang-format][docfix] Update predefined styles in docs

[JakeMerdichAMD]

Summary

The predefined styles that clang-format supports are listed in two
places, and neither is up-to-date. GNU style isn't mentioned at all!

[curdeius]

Nice!

clang/docs/ClangFormatStyleOptions.rst

* ``Mozilla``
     A style complying with `Mozilla's style guide
     <https://developer.mozilla.org/en-US/docs/Developer_Guide/Coding_Style>`_
   * ``WebKit``
     A style complying with `WebKit's style guide
     <https://www.webkit.org/coding/coding-style.html>`_
   * ``Microsoft``
     A style complying with `Microsoft's style guide
     <https://docs.microsoft.com/en-us/visualstudio/ide/editorconfig-code-style-settings-reference?view=vs-2017>`_
+  * ``GNU``
+    A style complying with the `GNU coding standards
+    <https://www.gnu.org/prep/standards/standards.html>`_
 
 .. START_FORMAT_STYLE_OPTIONS
 
 **AccessModifierOffset** (``int``)
   The extra indent or outdent of access modifiers, e.g. ``public:``.
 
 **AlignAfterOpenBracket** (``BracketAlignmentStyle``)
   If ``true``, horizontally aligns arguments after an open bracket.

clang/docs/LibFormat.rst

 
 .. _style-options:
 
 Style Options
 -------------
 
 The style options describe specific formatting options that can be used in
 order to make `ClangFormat` comply with different style guides. Currently,
-two style guides are hard-coded:
+several style guides are hard-coded:
 
 .. code-block:: c++
 
   /// Returns a format style complying with the LLVM coding standards:
   /// https://llvm.org/docs/CodingStandards.html.
   FormatStyle getLLVMStyle();
 
   /// Returns a format style complying with Google's C++ style guide:
   /// http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml.
   FormatStyle getGoogleStyle();
 
+  /// Returns a format style complying with Chromium's style guide:
+  /// https://chromium.googlesource.com/chromium/src/+/master/styleguide/styleguide.md
+  FormatStyle getChromiumStyle();
+
+  /// Returns a format style complying with the GNU coding standards:
+  /// https://www.gnu.org/prep/standards/standards.html
+  FormatStyle getGNUStyle();
+
+  /// Returns a format style complying with Mozilla's style guide
+  /// https://firefox-source-docs.mozilla.org/code-quality/coding-style/index.html
+  FormatStyle getMozillaStyle();
+
+  /// Returns a format style complying with Webkit's style guide:
+  /// https://webkit.org/code-style-guidelines/
+  FormatStyle getWebkitStyle();
+
+  /// Returns a format style complying with Microsoft's style guide:
+  /// https://docs.microsoft.com/en-us/visualstudio/ide/editorconfig-code-style-settings-reference
+  FormatStyle getMicrosoftStyle();
+
 These options are also exposed in the :doc:`standalone tools <ClangFormat>`
 through the `-style` option.
 
 In the future, we plan on making this configurable.