Improve doc for CustomConverter in gatsby-transformer-asciidoc (#27512)

This patch makes a correction to the actual CustomConverter section of
the documentation in gatsby-transformer-asciidoc. It was referenced by
as TemplateConverter, but is in fact a CustomConverter.
This patch also extends the doc regarding the actual usage and closes #27511.

packages/gatsby-transformer-asciidoc/README.md

@@ -154,30 +154,30 @@ Each attribute with the prefix page- will be automatically added under `pageAttr
 }
 ```
 
-## Define a custom converter
+## Define a Custom Converter
 
 You can define a custom converter by adding the `converterFactory` option.
 
 ```javascript
-// In your gatsby-config.js, make sure to import or declare TemplateConverter
+// In your gatsby-config.js, make sure to import or declare CustomConverter
 plugins: [
   {
     resolve: `gatsby-transformer-asciidoc`,
     options: {
-      converterFactory: TemplateConverter,
+      converterFactory: CustomConverter,
     },
   },
 ]
 ```
 
-`TemplateConverter` is a custom javascript class you'll need to create. Information on how to write a custom `TemplateConverter` can be found at the [asciidoctor docs](https://asciidoctor-docs.netlify.app/asciidoctor.js/extend/converter/custom-converter/).
+`CustomConverter` is a custom javascript class you'll need to create. Information on how to write a custom `CustomConverter` can be found at the [asciidoctor docs](https://asciidoctor-docs.netlify.app/asciidoctor.js/extend/converter/custom-converter/).
 
 In the example below, we will use a custom converter to convert paragraphs but the other nodes will be converted using the built-in HTML5 converter:
 
 ```javascript
 const asciidoc = require(`asciidoctor`)()
 
-class TemplateConverter {
+class CustomConverter {
   constructor() {
     this.baseConverter = asciidoc.Html5Converter.$new()
   }
@@ -191,3 +191,6 @@ class TemplateConverter {
   }
 }
 ```
+
+gatsby-transformer-asciidoc takes then this class, **not** a instance of `CustomConverter`, as the `converterFactory` option.
+You can also reuse the internal converter of gatsby-transformer-asciidoc, since the constructor of a given `CustomConverter` will be call with it as parameter.