Document the use of GIT_ASKPASS and SSH_ASKPASS.

BUG=
R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/18851004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@211148 0039d316-1c4b-4281-b951-d872f2087c98