Document token registration

[Twi1ightSparkle]

If registration_requires_token: true in the Synapse config, then Element supports token registration. Kind of... the process isn't very nice/obvious. Document how a user would do this https://matrix-org.github.io/synapse/latest/usage/administration/admin_api/registration_tokens.html

[Twi1ightSparkle]

So I don't forget, the process is

• /#/register
• enter user, password, and optionally email
• Next
• Click the small "Start authentication" button
• Enter the registration token in the window that opens, and "Authenticate"
• Continue as normal