NaCl documentation: nits on ARM sandbox documentation

R= sbc@chromium.org, binji@chromium.org, awatson@chromium.org
TEST= none
BUG= none
NOTRY=true
(documentation only change)

Review URL: https://codereview.chromium.org/173343004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252863 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252863