adding kdoc preprocessing guide

KDOC_PREPROCESSING.md

@@ -321,6 +321,28 @@ like `@file:ExcludeFromSources`.
 
 ## KDoc Preprocessor Conventions in DataFrame
 
+TODO
+
 ## KDoc -> WriterSide
 
-### `@ExportAsHtml`
+There's a special annotation, `@ExportAsHtml`, that allows you to export the content of the KDoc of the annotated
+function, interface, or class as HTML.
+The MarkDown of the KDoc is rendered to HTML using [JetBrains/markdown](https://github.com/JetBrains/markdown) and, in
+the case of DataFrame, put in [./docs/StardustDocs/snippets/kdocs](./docs/StardustDocs/snippets/kdocs).
+From there, the HTML can be included in any WriterSide page as an iFrame.
+This can be done using our custom `<dataFrame src=""/>` tag.
+
+An example of the result can be found in the
+[DataFrame documentation](https://kotlin.github.io/dataframe/columnselectors.html#full-dsl-grammar).
+
+The annotation supports two parameters: `theme`, and `stripReferences`, which both are `true` by default. 
+When the `theme` argument is `true`, some CSS is added to the HTML output to make it look good in combination with
+WriterSide. If the `stripReferences` is `true`, all `[]` references are stripped,
+like `[name][fully.qualified.name]` -> `<code>name</code>`. This makes the output a lot more readable since
+the references won't be clickable in the HTML output anyway.
+
+Optionally, the tags `@exportAsHtmlStart` and `@exportAsHtmlEnd` can be used to mark the start and end of the content
+to be exported as HTML.
+This is useful when you only want to export a part of the KDoc.
+
+`@ExportAsHtml` can also safely be used in combination with `@ExcludeFromSources`.