Improve doc for CustomConverter in gatsby-transformer-asciidoc (#27512)
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.
gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby