docs(check-user): add more hints and details about getent

subjects/devops/check-user/README.md

@@ -30,10 +30,16 @@ $
 ### Error handling
 
 All errors will print a specific message on **stderr** (ending with a newline) and returns a specific non-zero value:
-- Wrong number of arguments: `"Error: expect 2 arguments"`, returns `1`.
+- Wrong number of arguments: `"Error: expect 2 arguments"`, exit with `1`.
 - First argument different from `-e` or `-i`: `"Error: unknown flag"`, exit with `1`.
 
 ### Hints
+- `getent` is a command to get entries from a database. `passwd` is the database where information about users is stored.
+- `getent passwd` will give you the list of all users.
+- `getent passwd <username>` will give you information about a specific user.
+- If the user doesn't exists `getent` returns an empty string, use this at your advantage for `-e` flag.
 
-> `man getent` will be a great resource to explore  
+> `man getent` will provide extensive documentation about this command.
+
+### Resources
 > [List Linux users](https://linuxize.com/post/how-to-list-users-in-linux/)