vscode: add docs about syntax tree

docs/user/features.md

@@ -81,6 +81,12 @@ Join selected lines into one, smartly fixing up whitespace and trailing commas.
 Shows the parse tree of the current file. It exists mostly for debugging
 rust-analyzer itself.
 
+You can hover over syntax nodes in the opened text file to see the appropriate
+rust code that it refers to and the rust editor will also highlight the proper
+text range.
+
+<img src="https://user-images.githubusercontent.com/36276403/78043783-7425e180-737c-11ea-8653-b02b773c5aa1.png" alt="demo" height="200px" >
+
 #### Expand Macro Recursively
 
 Shows the full macro expansion of the macro at current cursor.