Document eval plugin limitations

- mention std[in|out|err] problems
- link to differences with doctest

Author: xsebek

docs/features.md

@@ -274,6 +274,14 @@ Evaluates code blocks in comments with a click. [Tutorial](https://github.com/ha
 
 ![Eval Demo](https://raw.githubusercontent.com/haskell/haskell-language-server/master/plugins/hls-eval-plugin/demo.gif)
 
+Known limitations:
+
+- Standard [input](https://github.com/haskell/haskell-language-server/issues/2913)
+  and [(error) output](https://github.com/haskell/haskell-language-server/issues/1977)
+  are not handled and can break HLS communication!
+- While similar to [doctest](https://hackage.haskell.org/package/doctest), some of its features are unsupported,
+  see [Differences with doctest](https://github.com/haskell/haskell-language-server/blob/master/plugins/hls-eval-plugin/README.md#differences-with-doctest).
+
 ### Make import lists fully explicit code lens
 
 Provided by: `hls-explicit-imports-plugin`