Make building with clang documentation a bit easier to find

BUG=skia:
NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=1495913002

Review URL: https://codereview.chromium.org/1495913002

site/user/quick/desktop.md

@@ -32,7 +32,8 @@ Gyp Options
 
 Skia relies on Gyp to generate build files.  Read about
 [specifying options for Gyp](/user/tips#gypdefines) to set the
-compile-time settings, compiler, build systems, and build directory.
+compile-time settings, compiler (e.g. use clang instead of gcc), build systems,
+and build directory.
 
 Run unit and correctness tests
 ------------------------------

site/user/quick/linux.md

@@ -81,6 +81,8 @@ Notes
     <!--?prettify lang=sh?-->
 
         GYP_DEFINES='skia_arch_type=x86' python bin/sync-and-gyp
+2.  By default, many Linux systems use gcc by default.  To use clang you will
+need to [set the CC and CXX environment variables](/user/tips#gypdefines).
 
 Generate build files
 --------------------

site/user/tips.md

@@ -20,14 +20,24 @@ Note: Setting enviroment variables in the Windows CMD.EXE shell [uses a
 different syntax](/user/quick/windows#env).
 
 You can also set environment variables such as `CC`, `CXX`,
-`CFLAGS`, or `CPPFLAGS` to control how Skia is compiled. For
-example:
+`CFLAGS`, `CXXFLAGS`, or `CPPFLAGS` to control how Skia is compiled.
+To build with clang, for example:
 
 <!--?prettify lang=sh?-->
 
     CC='clang' CXX='clang++' python bin/sync-and-gyp
     ninja -C out/Debug
 
+To build with clang and enable a compiler warning for unused parameters in C++
+(but not C or assembly) code:
+
+<!--?prettify lang=sh?-->
+
+    CXXFLAGS='-Wunused-parameter'
+    CC='clang' CXX='clang++' python bin/sync-and-gyp
+    ninja -C out/Debug
+
+
 The `GYP_GENERATORS` environment variable can be used to set the
 build systems that you want to use (as a comma-separated list).
 The default is `'ninja,msvs-ninja'` on Windows, `'ninja,xcode'` on