[NaCl Docs] Update the SDK download docs to mention pepper_31 + PNaCl

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3634
NOTRY=true

Review URL: https://codereview.chromium.org/26535002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228274 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228274