[BUG] API class descriptions / tag descriptions are not rendering

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

API class descriptions / tag descriptions are rendering as empty.

Take the official PHP template, which includes a class-level {{#description}} for its APIDoc template:

# {{invokerPackage}}\{{classname}}{{#description}}

{{.}}{{/description}}

All URIs are relative to {{basePath}}, except if the operation defines another base path.

{{#description}} always renders empty, despite having tag descriptions in the OpenAPI Spec.

openapi-generator version

6.2.1

OpenAPI declaration file content or URL

Use the official PetStore API Spec, which includes these tag descriptions:

openapi: 3.0.0
tags:
  - name: pet
    description: Everything about your Pets
  - name: store
    description: Access to Petstore orders
  - name: user
    description: Operations about user

Generation Details

Generate a PHP client:

openapi-generator-cli generate -g php \
  -i https://raw.githubusercontent.com/OpenAPITools/openapi-generator/master/modules/openapi-generator/src/test/resources/3_0/petstore.yaml \
  -o temp-php-client

See the generated result (docs/Api/PetApi.md):

# OpenAPI\Client\PetApi

All URIs are relative to http://petstore.swagger.io/v2, except if the operation defines another base path.

See the template (official PHP template):

# {{invokerPackage}}\{{classname}}{{#description}}

{{.}}{{/description}}

All URIs are relative to {{basePath}}, except if the operation defines another base path.

Notice how the PHP generator seems to support an API class-level description, yet doesn't render it (at least from the tag description... I'm not sure where else it would be reading this from?). I have a feeling many of the other generators exhibit this same behaviour (i.e. have a API class-level description but without any way of rendering content into it from the spec).

Steps to reproduce

See Generation Details (above).

Start with modules/openapi-generator/src/main/resources/php/api_doc.mustache and the official petstore.yaml spec. Generate the PHP client using the documented command and compare the tag descriptions with docs/Api/PetApi.md. Done means the API class or tag descriptions appear in the generated documentation as expected.