Fix def-env docs according to docs#761 (#7972)

See nushell/nushell.github.io#761

# User-Facing Changes

Docs reflect up-to-date reality

Co-authored-by: Canop <cano.petrole@gmail.com>

crates/nu-command/src/core_commands/def_env.rs

@@ -28,32 +28,7 @@ impl Command for DefEnv {
     fn extra_usage(&self) -> &str {
         r#"This command is a parser keyword. For details, check:
   https://www.nushell.sh/book/thinking_in_nu.html
-
-=== EXTRA NOTE ===
-All blocks are scoped, including variable definition and environment variable changes.
-
-Because of this, the following doesn't work:
-
-def-env cd_with_fallback [arg = ""] {
-    let fall_back_path = "/tmp"
-    if $arg != "" {
-        cd $arg
-    } else {
-        cd $fall_back_path
-    }
-}
-
-Instead, you have to use cd in the top level scope:
-
-def-env cd_with_fallback [arg = ""] {
-    let fall_back_path = "/tmp"
-    let path = if $arg != "" {
-        $arg
-    } else {
-        $fall_back_path
-    }
-    cd $path
-}"#
+"#
     }
 
     fn is_parser_keyword(&self) -> bool {