remove information about implementation details

Author: Qelxiros

docs/src/extensions.md

@@ -22,10 +22,9 @@ $ ls -w=80
 
 ## `env`
 
-GNU `env` allows the empty string to be used as an environment variable name. This is explicitly
-unsupported by the Rust standard library (see the
-[docs](https://doc.rust-lang.org/stable/std/env/fn.set_var.html) for more information), so uutils
-will show a warning on any such assignment.
+GNU `env` allows the empty string to be used as an environment variable name.
+This is unsupported by uutils, and it will show a warning on any such
+assignment.
 
  `env` has an additional `-f`/`--file` flag that can
 parse `.env` files and set variables accordingly. This feature is adopted from `dotenv` style