Update some clang plugin documentation.

docs/clang.md

 # Clang
 
 [Clang](http://clang.llvm.org/) is a compiler with many desirable features
 (outlined on their website).
 
 Chrome can be built with Clang. It is now the default compiler on Mac and Linux
 for building Chrome, and it is currently useful for its warning and error
 messages on Android and Windows.
 
 See
@@ skipping 33 matching lines @@
 http://groups.google.com/a/chromium.org/group/clang/topics
 
 ## Using plugins
 
 The
 [chromium style plugin](http://dev.chromium.org/developers/coding-style/chromium-style-checker-errors)
 is used by default when clang is used.
 
 If you're working on the plugin, you can build it locally like so:
 
-1.  Run `./tools/clang/scripts/update.sh --force-local-build --without-android`
+1.  Run `./tools/clang/scripts/update.py --force-local-build --without-android`
     to build the plugin.
-1.  Build with clang like described above.
+1.  Run `ninja -C third_party/llvm-build/Release+Asserts/` to build incrementally.
+1.  Build with clang like described above, but, if you use goma, disable it.
 
-TODO: writing_clang_plugins does not exist.
+To test the FindBadConstructs plugin, run:
+
+    (cd tools/clang/plugins/tests && \
+     ./test.sh ../../../../third_party/llvm-build/Release+Asserts/bin/clang \
+               ../../../../third_party/llvm-build/Release+Asserts/lib/libFindBadConstructs.so)
+
 To run [other plugins](writing_clang_plugins.md), add these to your
 `GYP_DEFINES`:
 
 *   `clang_load`: Absolute path to a dynamic library containing your plugin
 *   `clang_add_plugin`: tells clang to run a specific PluginASTAction
 
 So for example, you could use the plugin in this directory with:
 
 *   `GYP_DEFINES='clang=1 clang_load=/path/to/libFindBadConstructs.so
     clang_add_plugin=find-bad-constructs' gclient runhooks`
@@ skipping 49 matching lines @@
 
 You can then run `head out/Release/build.ninja` and check that the first to
 lines set `cc` and `cxx` to your clang binary. If things look good, run `ninja
 -C out/Release` to build.
 
 If your clang revision is very different from the one currently used in chromium
 
 *   Check `tools/clang/scripts/update.sh` to find chromium's clang revision
 *   You might have to tweak warning flags. Or you could set `werror=` in the
     line above to disable warnings as errors (but this only works on Linux).