Spell out password auth providers can be used for registration

This issue has been migrated from [#11800](https://github.com/matrix-org/synapse/issues/11800).

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Currently the documentation says that password auth providers provide additional features to authentication flows, but doesn't spell out that it can be used for registration. While this is technically correct (since password auth providers interact with User-Interactive Authentication in general, and registration uses UIA), it's not super clear that password auth providers can be used for registration, since "authentication" usually refers to authenticating existing users on the homeserver (vs registering new ones). We should update the documentation to spell out explicitly that they can be used for registration as well.