help_docs: Update stream-sending-policy help doc.

Uses new `select-stream-view-general.md` for instructions.

Also, clarifies location of button/icon as well as the
header for the privacy setting in the update modal.

Finally, updates save instruction to use `save-changes.md`.

templates/zerver/help/stream-sending-policy.md

@@ -13,11 +13,13 @@ certain users can send messages.
 
 1. Select a stream.
 
-1. On the right, click **[Change]** next to the description of the stream
-   permissions.
+{!select-stream-view-general.md!}
 
-1. Under "Who can post to the stream?", select the option you prefer.
+1. Click the **pencil** (<i class="fa fa-pencil"></i>)
+   to the right of **Stream permissions**.
 
-1. Click **Save changes**.
+1. Under **Who can post to the stream?**, select the option you prefer.
+
+{!save-changes.md!}
 
 {end_tabs}