Explain unclear semantics of property $ref in Path Item Object (OAI#1964)

* 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.

* Update versions/3.1.0.md

Co-authored-by: Ron <ron@swagger.io>
Signed-off-by: Mike Ralphson <mike.ralphson@gmail.com>

Author: 2 people

versions/3.1.0.md

@@ -740,7 +740,7 @@ 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).  In case a Path Item Object field appears both in the defined object and the referenced object, 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.