RDoc is the default documentation tool. RDoc syntax is enabled by default inside Ruby files and libraries.
You can find TextTile or Markdown used outside of the Ruby classes for text files, such as README or CHANGELOG files. But this is not standard syntax, so you need a way to tell the syntax where the file syntax is written, usually this path is the file extension.
GitHub, for example, follows this particular convention. If the .textile or .markdown extension is not specified, the file is parsed as an RDoc.
Remember: AFAIK, inline documentation is always parsed as an RDoc.