Update documentation for '>>' operator usage

Clarify the usage of the '>>' operator for appending output to files, including examples and test expectations.

Author: TropicolX

stage_descriptions/redirection-03-el9.md

@@ -1,33 +1,43 @@
 In this stage, you'll implement support for appending the output of a command to a file.
 
-### The `1>>` Operator
+### The `>>` Operator
 
-The `1>>` operator is used to append the output of a command to a file.
-As a special case, if the file descriptor is not specified before the operator `>>`, the output is redirected to the standard output by default, so `>>` is equivalent to `1>>`.
+The `>>` operator appends the standard output of a command to a file. Unlike `>`, which overwrites the file, `>>` adds the output to the end of the file and preserves any existing content.
 
-Learn more about [Appending Stdout](https://www.gnu.org/software/bash/manual/bash.html#Appending-Redirected-Output).
+If the file doesn't exist, it is created (just like `>`). If the file already exists, the new output is added to the end.
+
+For example:
+```bash
+$ echo first >> output.txt
+$ echo second >> output.txt
+$ cat output.txt
+first
+second
+```
+
+You can also explicitly write `1>>` to append to standard output. Both `>>` and `1>>` do exactly the same thing.
 
 ### Tests
 
 The tester will execute your program like this:
-
 ```bash
-./your_program.sh
+$ ./your_program.sh
 ```
 
-It will then send a series of commands to your shell, executing commands, and appending their output to a file:
-
+It will then send a series of commands to your shell and attempt to append their output to a file:
 ```bash
 $ ls /tmp/baz >> /tmp/bar/bar.md
 $ cat /tmp/bar/bar.md
 apple
 banana
 blueberry
+
 $ echo 'Hello Emily' 1>> /tmp/bar/baz.md
 $ echo 'Hello Maria' 1>> /tmp/bar/baz.md
 $ cat /tmp/bar/baz.md
 Hello Emily
 Hello Maria
+
 $ echo "List of files: " > /tmp/bar/qux.md
 $ ls /tmp/baz >> /tmp/bar/qux.md
 $ cat /tmp/bar/qux.md
@@ -37,5 +47,8 @@ banana
 blueberry
 ```
 
-The tester will check if the commands are executed correctly and append their output to a file as specified.
-The file contents will also be checked for correctness.
+The tester will verify that:
+- Commands with `>>` append their standard output to the specified file
+- Commands with `1>>` work identically to `>>`.
+- Existing file content is preserved (not overwritten).
+- Files are created if they don't exist.