Add docs about syntax highlighting differences

We use Monarch and Rogue libraries for syntax highlighting
but the docs lack any detail about Monarch.

doc/user/profile/preferences.md

@@ -63,7 +63,10 @@ Dark theme currently only works with the 'Dark' syntax highlighting.
 
 NOTE: **Note:**
 GitLab uses the [rouge Ruby library](http://rouge.jneen.net/ "Rouge website")
-for syntax highlighting. For a list of supported languages visit the rouge website.
+for syntax highlighting outside of any Editor context. The WebIDE (like Snippets)
+uses [Monaco Editor](https://microsoft.github.io/monaco-editor/) and it's provided [Monarch](https://microsoft.github.io/monaco-editor/monarch.html) library for
+syntax highlighting. For a list of supported languages, visit the documentation of
+the respective libraries.
 
 Changing this setting allows you to customize the color theme when viewing any
 syntax highlighted code on GitLab.

doc/user/project/highlighting.md

 # Syntax Highlighting
 
-GitLab provides syntax highlighting on all files and snippets through the [Rouge](https://rubygems.org/gems/rouge) rubygem. It will try to guess what language to use based on the file extension, which most of the time is sufficient.
+GitLab provides syntax highlighting on all files through the [Rouge](https://rubygems.org/gems/rouge) Ruby gem. It will try to guess what language to use based on the file extension, which most of the time is sufficient.
+
+NOTE: **Note:**
+The [Web IDE](web_ide/index.md) and [Snippets](../snippets.md) use [Monaco Editor](https://microsoft.github.io/monaco-editor/)
+for text editing, which internally uses the [Monarch](https://microsoft.github.io/monaco-editor/monarch.html)
+library for syntax highlighting.
 
 If GitLab is guessing wrong, you can override its choice of language using the `gitlab-language` attribute in `.gitattributes`. For example, if you are working in a Prolog project and using the `.pl` file extension (which would normally be highlighted as Perl), you can add the following to your `.gitattributes` file:
 
@@ -27,3 +32,6 @@ To disable highlighting entirely, use `gitlab-language=text`. Lots more fun shen
 ```
 
 Please note that these configurations will only take effect when the `.gitattributes` file is in your default branch (usually `master`).
+
+NOTE: **Note:**
+The Web IDE does not support `.gitattribute` files, but it's [planned for a future release](https://gitlab.com/gitlab-org/gitlab/-/issues/22014).

doc/user/project/web_ide/index.md

@@ -38,7 +38,10 @@ The Web IDE currently provides:
 
 Because the Web IDE is based on the [Monaco Editor](https://microsoft.github.io/monaco-editor/),
 you can find a more complete list of supported languages in the
-[Monaco languages](https://github.com/Microsoft/monaco-languages) repository.
+[Monaco languages](https://github.com/Microsoft/monaco-languages) repository. Under the hood,
+Monaco uses the [Monarch](https://microsoft.github.io/monaco-editor/monarch.html) library for syntax highlighting.
+
+If you are missing Syntax Highlighting support for any language, we prepared a short guide on how to [add support for a missing language Syntax Highlighting.](https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/ide/lib/languages/README.md)
 
 NOTE: **Note:**
 Single file editing is based on the [Ace Editor](https://ace.c9.io).