Update the link to "Your Guard note may not change"

Currently, when you click on the circuit display in Tor Browser, the text "Your Guard note may not change. Learn more" points to https://tb-manual.torproject.org/en-US/ (adjusting for the locale).

This is not very specific or helpful so we should instead point to https://support.torproject.org/tbb/tbb-2/ which addresses this question in better detail.