Clarify language-servers documentation for mergeable LSP features (`d…

…iagnostics`, `code-action`, `completion`, `document-symbols` and `workspace-symbols`)

book/src/languages.md

@@ -168,6 +168,7 @@ language-servers = [ { name = "typescript-language-server", except-features = [
 
 Each requested LSP feature is prioritized in the order of the `language-servers` array.
 For example the first `goto-definition` supported language server (in this case `typescript-language-server`) will be taken for the relevant LSP request (command `goto_definition`).
+The features `diagnostics`, `code-action`, `completion`, `document-symbols` and `workspace-symbols` are an exception to that rule, as they are working for all language servers at the same time and are merged together, if enabled for the language.
 If no `except-features` or `only-features` is given all features for the language server are enabled.
 If a language server itself doesn't support a feature the next language server array entry will be tried (and so on).