Document what are access keys and how to translate them

We are having issues with the Access Keys for the BridgeDB options page. Translators do not realise what are they and how they work. Plus, they repeat the same access key for different options.

Strings that use access keys at the moment are: '_Just give me bridges', 'Do you need a Pluggable _Transport?' and '_Yes'.

Once we have a proper explanation we can ask BridgeDB devs to add it or link it to the translator's comments.