NaCl docs: update computed goto support

mseaborn added computed goto lowering in:
https://codereview.chromium.org/120773002

R= binji@chromium.org, sbc@chromium.org, mseaborn@chromium.org
TEST=none
BUG= https://code.google.com/p/nativeclient/issues/detail?id=3695
NOTRY=true
(documentation only change)

Review URL: https://codereview.chromium.org/185653007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254570 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254570