Document Google OpenID Connect email attribute (#14081)

changelog.d/14081.doc

+Expand Google OpenID Connect example config to map email attribute. Contributed by @ptman.

docs/openid.md

@@ -336,11 +336,12 @@ oidc_providers:
        issuer: "https://accounts.google.com/"
        client_id: "your-client-id" # TO BE FILLED
        client_secret: "your-client-secret" # TO BE FILLED
-       scopes: ["openid", "profile"]
+       scopes: ["openid", "profile", "email"] # email is optional, read below
        user_mapping_provider:
          config:
            localpart_template: "{{ user.given_name|lower }}"
            display_name_template: "{{ user.name }}"
+           email_template: "{{ user.email }}" # needs "email" in scopes above
    ```
 4. Back in the Google console, add this Authorized redirect URI: `[synapse
    public baseurl]/_synapse/client/oidc/callback`.