[Backport release-24.05] nixos/doc: update Installing section (#342125

)

nixos/doc: update `Installing` section (#341995)

Mention how to set a user password right after installation.
Also don't suggest to use `useradd` to add users.

(cherry picked from commit 4c1d538)

Co-authored-by: misuzu <[email protected]>

nixos/doc/manual/installation/installing.chapter.md

@@ -500,6 +500,12 @@ Use the following commands:
     Retype new password: ***
     ```
 
+    If you have a user account declared in your `configuration.nix` and plan to log in using this user, set a password before rebooting, e.g. for the `alice` user:
+
+    ```ShellSession
+    # nixos-enter --root /mnt -c 'passwd alice'
+    ```
+
     ::: {.note}
     For unattended installations, it is possible to use
     `nixos-install --no-root-passwd` in order to disable the password
@@ -519,15 +525,13 @@ Use the following commands:
     menu. This allows you to easily roll back to a previous
     configuration if something goes wrong.
 
-    You should log in and change the `root` password with `passwd`.
-
-    You'll probably want to create some user accounts as well, which can
-    be done with `useradd`:
+    Use your declared user account to log in.
+    If you didn’t declare one, you should still be able to log in using the `root` user.
 
-    ```ShellSession
-    $ useradd -c 'Eelco Dolstra' -m eelco
-    $ passwd eelco
-    ```
+    ::: {.note}
+    Some graphical display managers such as SDDM do not allow `root` login by default, so you might need to switch to TTY.
+    Refer to [](#sec-user-management) for details on declaring user accounts.
+    :::
 
     You may also want to install some software. This will be covered in
     [](#sec-package-management).