Update GN documentation.

tools/gn/docs/faq.md

 # GN Frequently Asked Questions
 
 [TOC]
 
-## How will the build be converted?
-
-We intend to build a second independent build in parallel to the GYP
-build. Previous efforts to generate GYP as an intermediate stage proved
-difficult. There will be some smaller set of bots compiling this build,
-and we'll keep the GN build running on these configurations.
-
-## What is unsupported in GN?
-
-The main features not supported in GN yet are:
-  * Mac/iOS bundles
-
 ## Where is the GN documentation?
 
-Rather than on a separate wiki, it is versioned with the tool. Run `gn
-help`. See also the [quick start](quick_start.md) guide and the
-[language and operation details](language.md).
+GN has extensive built-in help, so you can run `gn help`, but you can
+also see all of the help on [the reference page](reference.md). See
+also the [quick start](quick_start.md) guide and the [language and
+operation details](language.md).
 
-## What is likely to break?
+## Can I generate XCode or Visual Studio projects?
 
-Since common.gypi is not used for GN-generated GYP files, any rules
-there will no longer apply. There is a _lot_ of logic in there for many
-build configurations and various conditions where certain flags should
-or should not be used. Some of these build configurations aren't even
-supported any more. Some are run on other waterfalls or are used by
-individuals manually setting GYP\_DEFINES on their local system.
+You can generate skeleton (or wrapper) projects for Xcode, Visual Studio,
+QTCreator, and Eclipse that will list the files and targets in the
+build, but use Ninja to do the actual build. You cannot generate "real"
+projects that look like native ones like GYP could.
 
-## Will XCode/Visual Studio still be supported?
-
-Visual Studio is supported. Visual Studio can be used as an IDE for code
-browsing or debugging but Ninja is used for building.
 Run `gn help gen` for more details.
 
-XCode is not supported yet. We need help!
-
-## I'm weird. Will my uncommon build mode be supported?
-
-One of the main benefits of the build changeover is that it will
-encourage us to refactor the build system. The project has generally not
-been as strict with build complexity and maintenance as we have with
-source code, and a lot of cruft has accumulated.
-
-In most cases, we will want to rethink how build flags are supported. We
-want to be more modular rather than throwing everything in the
-common.gypi equivalent. The bar for additions at this level will be very
-high, and we will need to figure out how to design certain build
-features. If you are interested in some weird configurations, this will
-likely make your life more difficult in the short term, but will
-hopefully bring long-term benefits for everybody.
-
-In some cases, we may decide that the overhead of supporting your build
-for BeOS running on a DEC Alpha is not in the interests of the project.
-There will likely be discussions about where to draw the line, and how
-to allow those who want to do weird things to do them cleanly without
-negatively affecting the rest of the Chromium community.
-
-## I'm only a little bit weird, will my development build flag be supported?
-
-Only if you do it yourself!
-
-Some features have build flags that turn on a debugging mode or switch
-between internal/external builds. This can be supported, but  as with
-GYP, your team will have to add an maintain the support.
-
-## I use supplement.gypi, what's the GN equivalent?
-
-Some projects use files called `supplement.gypi` to set build flags. GYP
-looks in each directory under `src` and adds merges these files into the
-build. The model is that then adding something to your gclient to add
-something to your build (e.g `src-internal`) automatically sets flags.
-
-This behavior is fragile and mysterious. Some people get build flags and
-they don't know why. If you remove the entry from your `.gclient` and
-don't remove the directory you'll be stuck on an old version of the
-flags/code and not know why. You can't have builds in the same checkout
-with the corresponding flags on and off. Some people and projects were
-abusing this behavior.
-
-In GN, such things should be done with build arguments (`gn args`) and
-configured on your build directory when you set it up. For some people,
-this will be an extra step. But it is explicit and clear, and you can
-have multiple builds in the same checkout with different flags.
-
 ## How do I generate common build variants?
 
 In GN, args go with a build directory rather than being global in the
 environment. To edit the args for your `out/Default` build directory:
 
 ```
 gn args out/Default
 ```
 
 You can set variables in that file:
 
   * The default is a debug build. To do a release build add
     `is_debug = false`
   * The default is a static build. To do a component build add
     `is_component_build = true`
   * The default is a developer build. To do an official build, set
     `is_official_build = true`
   * The default is Chromium branding. To do Chrome branding, set
     `is_chrome_branded = true`
 
 ## How do I do cross-compiles?
 
 GN has robust support for doing cross compiles and building things for
 multiple architectures in a single build.
 
 See [GNCrossCompiles](cross_compiles.md) for more info.
+
+## Can I control what targets are built by default?
+
+Yes! If you create a group target called "default" in the top-level (root)
+build file, i.e., "//:default", GN will tell Ninja to build that by
+default, rather than building everything.