Added documentation for sdk/self, property uri

I saw a reference to self's uri property in the documentation for [sdk/passwords](https://addons.mozilla.org/en-US/developers/docs/sdk/1.14/modules/sdk/passwords.html#search(options\)), but it was not mentioned at [sdk/self](https://addons.mozilla.org/en-US/developers/docs/sdk/1.14/modules/sdk/self.html).

I've updated the documentation after looking at the source code of sdk/self.js

(For the record, this is the example I'm referring to, taken from [sdk/passwords](https://addons.mozilla.org/en-US/developers/docs/sdk/1.14/modules/sdk/passwords.html#search(options\)))

    function show_my_addon_passwords() {
      require("sdk/passwords").search({
       url: require("sdk/self").uri,