Make Markdown dialect to use findable in documentation

[sdruskat]

So I've found out that mdBook uses CommonMark via https://github.com/raphlinus/pulldown-cmark (via inspecting the Cargo.toml file), correct?

pulldown-cmark seems to implement some things that aren't in the CommonMark specs, like tables (unless I've overlooked them in the CM specs), which seem to follow https://michelf.ca/projects/php-markdown/extra/.

This is information that would be nice to have in the User Guide, especially for someone coming from other markdown dialects. I appreciate that this may be too specific for most use cases, so I probably wouldn't put it on the first page, but perhaps in the "Format" section?

I'd be happy to provide a PR if this suggestion finds any support.