More PNaCl-ization of the distributing doc. Clean-up of NaCl sections.

The NaCl sections were trimmed down considerably because they duplicate a lot
of the CWS and apps documentation. Pointing to said docs is better since this
is one place less to get out of date (like with apps v2, offline mode, etc that
were added recently).

BUG=None
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/27337004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228990 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228990