Clarify some documentation on SymmetricKey
Clarify some documentation on SymmetricKey

BUG=None
TEST=None

Review URL: http://codereview.chromium.org/5013001

git-svn-id: http://src.chromium.org/svn/trunk/src@66259 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: dc157f93cdb39c27e1acfa128bff3c7548ba66b6
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser