docs: channel object / $ref note. (asyncapi#826)

Co-authored-by: Lukasz Gornicki <[email protected]>

spec/asyncapi.md

@@ -595,7 +595,7 @@ Describes the operations available on a single channel.
 
 Field Name | Type | Description
 ---|:---:|---
-<a name="channelItemObjectRef"></a>$ref | `string` | Allows for a referenced definition of this channel item. The referenced structure MUST be in the form of a [Channel Item Object](#channelItemObject). In case a Channel Item Object field appears both in the defined object and the referenced object, the behavior is _undefined_. Resolution is done as defined by the [JSON Reference](https://tools.ietf.org/html/draft-pbryan-zyp-json-ref-03). <br/><br/>**Deprecated:** Usage of the `$ref` property has been deprecated.
+<a name="channelItemObjectRef"></a>$ref | `string` | Allows for a referenced definition of this channel item. The referenced structure MUST be in the form of a [Channel Item Object](#channelItemObject). In case a Channel Item Object field appears both in the defined object and the referenced object, the behavior is _undefined_. Resolution is done as defined by the [JSON Reference](https://tools.ietf.org/html/draft-pbryan-zyp-json-ref-03). <br/><br/>**Deprecated:** Using the $ref property with other properties is deprecated since the 2.3.0 version of the specification.
 <a name="channelItemObjectDescription"></a>description | `string` | An optional description of this channel item. [CommonMark syntax](https://spec.commonmark.org/) can be used for rich text representation.
 <a name="channelItemObjectServers"></a>servers | [`string`] | The servers on which this channel is available, specified as an optional unordered list of names (string keys) of [Server Objects](#serverObject) defined in the [Servers Object](#serversObject) (a map). If `servers` is absent or empty then this channel must be available on all servers defined in the [Servers Object](#serversObject).
 <a name="channelItemObjectSubscribe"></a>subscribe | [Operation Object](#operationObject) | A definition of the SUBSCRIBE operation, which defines the messages produced by the application and sent to the channel.