Update FAQ to explain world_readable only (#277)

Follow-up to #239

docs/faq.md

@@ -19,54 +19,54 @@ messages from any given date and day-by-day navigation.
 
 ## Why did the archive bot join my room?
 
-Only public Matrix rooms with `shared` or `world_readable` [history
-visibility](https://spec.matrix.org/latest/client-server-api/#room-history-visibility) are
-accessible in the Matrix Public Archive. In some clients like Element, the `shared`
-option equates to "Members only (since the point in time of selecting this option)" and
-`world_readable` to "Anyone" under the **room settings** -> **Security & Privacy** ->
-**Who can read history?**.
+Only Matrix rooms with `world_readable` [history
+visibility](https://spec.matrix.org/latest/client-server-api/#room-history-visibility)
+are accessible in the Matrix Public Archive and indexed by search engines.
 
 But the archive bot (`@archive:matrix.org`) will join any public room because it doesn't
-know the history visibility without first joining. Any room without `world_readable` or
-`shared` history visibility will lead a `403 Forbidden`. And if the public room is in
-the room directory, it will be listed in the archive but will still lead to a `403
-Forbidden` in that case.
+know the history visibility without first joining. Any room that doesn't have
+`world_readable` history visibility will lead a `403 Forbidden`.
 
 The Matrix Public Archive doesn't hold onto any data (it's
 stateless) and requests the messages from the homeserver every time. The
 [archive.matrix.org](https://archive.matrix.org/) instance has some caching in place, 5
 minutes for the current day, and 2 days for past content.
 
-The Matrix Public Archive only allows rooms with `world_readable` history visibility to
-be indexed by search engines. See the [opt
-out](#how-do-i-opt-out-and-keep-my-room-from-being-indexed-by-search-engines) topic
-below for more details.
-
-### Why does the archive user join rooms instead of browsing them as a guest?
-
-Guests require `m.room.guest_access` to access a room. Most public rooms do not allow
-guests because even the `public_chat` preset when creating a room does not allow guest
-access. Not being able to view most public rooms is the major blocker on being able to
-use guest access. The idea is if I can view the messages from a Matrix client as a
-random user, I should also be able to see the messages in the archive.
-
-Guest access is also a much different ask than read-only access since guests can also
-send messages in the room which isn't always desirable. The archive bot is read-only and
-does not send messages.
+See the [opt out
+section](#how-do-i-opt-out-and-keep-my-room-from-being-indexed-by-search-engines) below
+for more details.
 
 ## How do I opt out and keep my room from being indexed by search engines?
 
-Only public Matrix rooms with `shared` or `world_readable` history visibility are
-accessible to view in the Matrix Public Archive. But only rooms with history visibility
-set to `world_readable` are indexable by search engines.
+Only Matrix rooms with `world_readable` [history
+visibility](https://spec.matrix.org/latest/client-server-api/#room-history-visibility)
+are accessible in the Matrix Public Archive and indexed by search engines. One easy way
+to opt-out is to change your rooms history visibility to something else if you don't
+intend for your room be world readable.
 
-Also see https://github.com/matrix-org/matrix-public-archive/issues/47 to track better
-opt out controls.
+Dedicated opt-out controls are being tracked in
+[#47](https://github.com/matrix-org/matrix-public-archive/issues/47).
 
-As a workaround for [archive.matrix.org](https://archive.matrix.org/) today, you can ban
-the `@archive:matrix.org` user if you don't want your room content to be shown in the
+As a workaround for [archive.matrix.org](https://archive.matrix.org/), you can ban the
+`@archive:matrix.org` user if you don't want your room content to be shown in the
 archive at all.
 
+### Why does the archive user join rooms instead peeking in the room or using guests?
+
+Since the archive only displays rooms with `world_readable` history visibility, we could
+peek into the rooms without joining. This is being explored in
+[#272](https://github.com/matrix-org/matrix-public-archive/pull/272). But peeking
+doesn't work when the server doesn't know about the room already (this is commonly
+referred to as federated peeking) which is why we have to fallback to joining the room
+in any case. We could solve the federated peeking problem and avoid the join with
+[MSC3266 room summaries](https://github.com/matrix-org/matrix-spec-proposals/pull/3266)
+to check whether the room is `world_readable` even over federation.
+
+Guests are completely separate concept and controlled by the `m.room.guest_access` state
+event in the room. Guest access is also a much different ask than read-only access since
+guests can also send messages in the room which isn't always desirable. The archive bot
+is read-only and does not send messages.
+
 ## Technical details
 
 The main readme has a [technical overview](../README.md#technical-overview) of the