help: Clarify documentation on guest users and streams.

This tries to make it more immediately clear facts like that guests
cannot see public streams.

templates/zerver/help/roles-and-permissions.md

@@ -15,8 +15,10 @@ There are several possible roles in a Zulip organization.
   organization to restrict the permissions of **New Members**; Members
   who do not have those restrictions are called **Full Members**.
 
-* **Guest**: Can only access streams they've been added to. Cannot create
+* **Guest**: Can only view or access streams they've been added to.
-  new streams or invite other users.
+  Guest users interact with public streams as though they were private
+  streams with shared history.  Cannot create new streams or invite
+  other users.
 
 * **Billing Administrator**: The user who upgrades the organization to
   a paid plan is, in addition to their normal role, a Billing

templates/zerver/help/stream-permissions.md

@@ -3,11 +3,13 @@
 Streams are similar to chatrooms, IRC channels, or email lists in that they
 determine who receives a message. There are three types of streams in Zulip.
 
-* **Public**: Anyone other than guests can join, and anyone (other
+* **Public**: Members can join and view the complete message history.
-  than guests) can view the complete message history without joining.
+  Guest users interact with public streams as though they were a
+  private stream with shared history.
 
-* **Private, shared history**: You must be added by a member of the stream. The
+* **Private, shared history**: You must be added by a member of the
-  complete message history is available as soon as you are added.
+  stream. The complete message history is available as soon as you are
+  added.
 
 * **Private, protected history**: You must be added by a member of the
   stream. You only have access to the messages that are sent after you are added.
@@ -29,8 +31,9 @@ At a high level:
   whether a stream with that name exists, but cannot see any other
   details about the stream.
 
-* From the perspective of a guest, all streams are private streams, and they
+* From the perspective of a guest, all streams are private streams,
-  additionally can't add other members to the streams they are subscribed to.
+  and they additionally can't add other members to the streams they
+  are subscribed to.
 
 There are two situations in which an organization owner or
 administrator can access private stream messages:
@@ -49,6 +52,7 @@ administrator can access private stream messages:
 
 |                       | Owners and admins | Members   | Guests
 |---                    |---                |---        |---
+| View stream name      | ✔          | ✔  | ◾
 | Join                  | ✔          | ✔  |
 | Unsubscribe           | ◾           | ◾   | ◾
 | Add others            | ✔          | ✔  |
@@ -76,6 +80,7 @@ subscribed.
 
 |                       | Owners and admins | Members   | Guests
 |---                    |---                |---        |---
+| View stream name      | ✔          | ◾   | ◾
 | Join                  |                   |           |
 | Unsubscribe           | ◾           | ◾   | ◾
 | Add others            | ◾           | ◾   |