Path Templating Clarification - proposed fix for #1830. (#1831)

* Proposed fix for #1830. Each variable expression in a path must have a corresponding path parameter.

* #1830 - Removed 'at least once' to defer the question about repeated references to a single path parameter.

* Update #1830 fix with suggestion from Darrel

@darrelmiller suggestions we use "template expression" instead of "variable expression" to align with RFC6570. Good idea.

versions/3.0.3.md

@@ -71,7 +71,9 @@ An OpenAPI definition can then be used by documentation generation tools to disp
 A document (or set of documents) that defines or describes an API. An OpenAPI definition uses and conforms to the OpenAPI Specification.
 
 ##### <a name="pathTemplating"></a>Path Templating
-Path templating refers to the usage of curly braces ({}) to mark a section of a URL path as replaceable using path parameters.
+Path templating refers to the usage of template expressions, delimited by curly braces ({}), to mark a section of a URL path as replaceable using path parameters.
+
+Each template expression in the path MUST correspond to a path parameter that is included in the [Path Item](#path-item-object) itself and/or in each of the Path Item's [Operations](#operation-object).
 
 ##### <a name="mediaTypes"></a>Media Types
 Media type definitions are spread across several resources.
@@ -1012,7 +1014,7 @@ There are four possible parameter locations specified by the `in` field:
 ##### Fixed Fields
 Field Name | Type | Description
 ---|:---:|---
-<a name="parameterName"></a>name | `string` | **REQUIRED**. The name of the parameter. Parameter names are *case sensitive*. <ul><li>If [`in`](#parameterIn) is `"path"`, the `name` field MUST correspond to the associated path segment from the [path](#pathsPath) field in the [Paths Object](#pathsObject). See [Path Templating](#pathTemplating) for further information.<li>If [`in`](#parameterIn) is `"header"` and the `name` field is `"Accept"`, `"Content-Type"` or `"Authorization"`, the parameter definition SHALL be ignored.<li>For all other cases, the `name` corresponds to the parameter name used by the [`in`](#parameterIn) property.</ul>
+<a name="parameterName"></a>name | `string` | **REQUIRED**. The name of the parameter. Parameter names are *case sensitive*. <ul><li>If [`in`](#parameterIn) is `"path"`, the `name` field MUST correspond to a template expression occurring within the [path](#pathsPath) field in the [Paths Object](#pathsObject). See [Path Templating](#pathTemplating) for further information.<li>If [`in`](#parameterIn) is `"header"` and the `name` field is `"Accept"`, `"Content-Type"` or `"Authorization"`, the parameter definition SHALL be ignored.<li>For all other cases, the `name` corresponds to the parameter name used by the [`in`](#parameterIn) property.</ul>
 <a name="parameterIn"></a>in | `string` | **REQUIRED**. The location of the parameter. Possible values are `"query"`, `"header"`, `"path"` or `"cookie"`.
 <a name="parameterDescription"></a>description | `string` | A brief description of the parameter. This could contain examples of use. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation.
 <a name="parameterRequired"></a>required | `boolean` | Determines whether this parameter is mandatory. If the [parameter location](#parameterIn) is `"path"`, this property is **REQUIRED** and its value MUST be `true`. Otherwise, the property MAY be included and its default value is `false`.