PNaCl docs: update to reflect libc++ and exception handling support

See recent PSA on the mailing list:
  https://groups.google.com/forum/#!topic/native-client-discuss/0spfg6O04FM

BUG= http://code.google.com/p/nativeclient/issues/detail?id=3623
R= dschuff@chromium.org
NOTRY=true
(documentation update)

Review URL: https://codereview.chromium.org/111303002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239792 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239792