[docs] Remind the user that password resets don't work without restarting. (#3998)

Every time I reset the password, I assume that something went wrong,
because I forget that the server will not pick up on the reset
password until it's been restarted according to the docs:

https://docs.gotosocial.org/en/latest/admin/cli/#gotosocial-admin-account-password

A reminder here would help reduce confusion.

cmd/gotosocial/action/admin/account/account.go

@@ -383,6 +383,7 @@ var Password action.GTSAction = func(ctx context.Context) error {
 	}
 
 	user.EncryptedPassword = string(encryptedPassword)
+	log.Info(ctx, "Updating password; you must restart the server to use the new password.")
 	return state.DB.UpdateUser(
 		ctx, user,
 		"encrypted_password",