Explain unclear semantics of property $ref in Path Item Object

Currently, as explained in OAI#1038 (comment) the description of `$ref` in [Path Item Object](https://github.com/OAI/OpenAPI-Specification/blob/3.0.2/versions/3.0.2.md#pathItemObject) is unclear about the semantics behing it. I took the explaination from issue OAI#1038 to make it more clear.

Author: cebe

versions/3.0.3.md

@@ -732,7 +732,7 @@ The path itself is still exposed to the documentation viewer but they will not k
 
 Field Name | Type | Description
 ---|:---:|---
-<a name="pathItemRef"></a>$ref | `string` | Allows for an external definition of this path item. The referenced structure MUST be in the format of a [Path Item Object](#pathItemObject). If there are conflicts between the referenced definition and this Path Item's definition, the behavior is *undefined*.
+<a name="pathItemRef"></a>$ref | `string` | Allows for an external definition of this path item. The referenced structure MUST be in the format of a [Path Item Object](#pathItemObject). A Path Item Object containing a `$ref` property is **not** a [Reference Object](#referenceObject). The properties of the referenced structure are merged with the local Path Item Object. If the same property exists in both, the referenced structure and the local one, this is a conflict. In this case the behavior is *undefined*.
 <a name="pathItemSummary"></a>summary| `string` | An optional, string summary, intended to apply to all operations in this path.
 <a name="pathItemDescription"></a>description | `string` | An optional, string description, intended to apply to all operations in this path. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation.
 <a name="pathItemGet"></a>get | [Operation Object](#operationObject) | A definition of a GET operation on this path.