api docs: Explain clearly how to create new streams.

Tweaked significantly by tabbott to cover this more clearly.

templates/zerver/api/add-subscriptions.md

@@ -4,6 +4,10 @@ Subscribe one or more users to one or more streams.
 
 `POST {{ api_url }}/v1/users/me/subcriptions`
 
+If any of the specified streams do not exist, they are automatically
+created, and configured using the `invite_only` setting specified in
+the arguments (see below).
+
 ## Usage examples
 <div class="code-section" markdown="1">
 <ul class="nav">

templates/zerver/api/arguments.json

@@ -216,7 +216,7 @@
         },
         {
             "argument": "invite_only",
-            "description": "A boolean specifying whether the streams specified in `subscriptions` are private or not. Default is `False`.",
+            "description": "A boolean specifying whether any streams created by this request should be created as private streams. Default is `False`.",
             "required": false,
             "example": "`True` or `False`"
         },

templates/zerver/api/create-stream.md

@@ -0,0 +1,7 @@
+# Create a stream
+
+You can create a stream using Zulip's REST API by submitting a
+[subscribe](/api/add-subscriptions) request with a stream name that
+doesn't yet exist.  You can specify the initial configuration of the
+stream using the `invite_only` and `announce` arguments to that
+request.

templates/zerver/help/include/rest-endpoints.md

@@ -13,6 +13,7 @@
 * [Add subscriptions](/api/add-subscriptions)
 * [Remove subscriptions](/api/remove-subscriptions)
 * [Get topics in a stream](/api/get-stream-topics)
+* [Create a stream](/api/create-stream)
 
 #### Users