docs: describe mutually exclusive create workspace template fields (#12834)

* docs: describe mutually exclusive create workspace template fields

Ideally we could do this in the OpenAPI spec, but there is no first
class "mutually exclusive" feature in OpenAPI. So in lieu of something
more complex, or changing our struct/validation, a description comment
should suffice.

* chore: Add description to code sample as well

docs/api/schemas.md

@@ -1646,6 +1646,8 @@ AuthorizationObject can represent a "set" of objects, such as: all workspaces in
 }
 ```
 
+CreateWorkspaceRequest provides options for creating a new workspace. Only one of TemplateID or TemplateVersionID can be specified, not both. If TemplateID is specified, the active version of the template will be used.
+
 ### Properties
 
 | Name                    | Type                                                                          | Required | Restrictions | Description                                                                                             |

docs/api/workspaces.md

@@ -14,6 +14,11 @@ curl -X POST http://coder-server:8080/api/v2/organizations/{organization}/member
 
 `POST /organizations/{organization}/members/{user}/workspaces`
 
+Create a new workspace using a template. The request must
+specify either the Template ID or the Template Version ID,
+not both. If the Template ID is specified, the active version
+of the template will be used.
+
 > Body parameter
 
 ```json