docs: Update docs/overview/ files to use channel.

Updates descriptive text that refer to Zulip channels in the
`docs/overview` files to use channel instead of stream, except for
the changelog file.

Part of the stream to channel rename project.

docs/overview/architecture-overview.md

@@ -50,7 +50,7 @@ numerous dedicated [Zulip API](https://zulip.com/api/) clients
 A server can host multiple Zulip _realms_ (organizations), each on its
 own (sub)domain. While most installations host only one organization, some
 host thousands, such as zulip.com. Each organization is a private
-chamber with its own users, streams, customizations, and so on. This
+chamber with its own users, channels, customizations, and so on. This
 means that one person might be a user of multiple Zulip realms. The
 administrators of an organization have a great deal of control over
 who can register an account, what permissions new users have, etc. For
@@ -281,7 +281,7 @@ self-explanatory names.
 - **ellipsis**: A small vertical three dot icon (technically called
   as ellipsis-v), present in sidebars as a menu icon.
   It offers contextual options for global filters (Combined feed
-  and Starred messages), stream filters and topics in left
+  and Starred messages), channel filters and topics in left
   sidebar and users in right sidebar. To avoid visual clutter
   ellipsis only appears in the web UI upon hover.
 
@@ -296,7 +296,7 @@ self-explanatory names.
 - **realm**: What the codebase calls an "organization" in the UI.
 
 - **recipient bar**: A visual indication of the context of a message
-  or group of messages, displaying the stream and topic or direct
+  or group of messages, displaying the channel and topic or direct
   message recipient list, at the top of a group of messages. A
   typical 1-line message to a new recipient shows to the user as
   three lines of content: first the recipient bar, second the