Document tools/mac/dump-static-initializers.py more.

BUG=none
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/27151005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228971 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228971