nix-env: improve page (#2807)

- add example showing installed packages
- replace status example with one for searching within installed packages
- improve description and token name in the "query available" example

pages/common/nix-env.md

@@ -2,13 +2,17 @@
 
 > Manipulate or query Nix user environments.
 
-- Show available package with name or without name:
+- List all installed packages:
 
-`nix-env -qa {{pkg_name}}`
+`nix-env -q`
 
-- Show the status of available packages:
+- Query installed packages:
 
-`nix-env -qas`
+`nix-env -q {{search_term}}`
+
+- Query available packages:
+
+`nix-env -qa {{search_term}}`
 
 - Install package: