docs: some improvements and clarifications

Signed-off-by: Nicola Murino <nicola.murino@gmail.com>

docs/groups.md

@@ -9,6 +9,8 @@ SFTPGo supports two types of groups:
 
 A user can be a member of a primary group and many secondary groups. Depending on the group type, the settings are inherited differently.
 
+:warning: SFTPGo groups are completely unrelated to system groups. Therefore, it is not necessary to add Linux/Windows groups to use SFTPGo groups.
+
 The following settings are inherited from the primary group:
 
 - home dir, if set for the group will replace the one defined for the user. The `%username%` placeholder is replaced with the username

docs/howto/getting-started.md

@@ -7,19 +7,19 @@ Virtual folders can be private or shared among multiple users, for shared virtua
 
 In this tutorial we explore the main features and concepts using the built-in web admin interface. Advanced users can also use the SFTPGo [REST API](https://sftpgo.stoplight.io/docs/sftpgo/openapi.yaml)
 
-- [Installation](#Installation)
-- [Initial configuration](#Initial-configuration)
-- [Creating users](#Creating-users)
-  - [Creating users with a Cloud Storage backend](#Creating-users-with-a-Cloud-Storage-backend)
-  - [Creating users with a local encrypted backend (Data At Rest Encryption)](#Creating-users-with-a-local-encrypted-backend-Data-At-Rest-Encryption)
-- [Virtual permissions](#Virtual-permissions)
-- [Virtual folders](#Virtual-folders)
-- [Configuration parameters](#Configuration-parameters)
-  - [Use PostgreSQL data provider](#Use-PostgreSQL-data-provider)
-  - [Use MySQL/MariaDB data provider](#Use-MySQLMariaDB-data-provider)
-  - [Use CockroachDB data provider](#Use-CockroachDB-data-provider)
-  - [Enable FTP service](#Enable-FTP-service)
-  - [Enable WebDAV service](#Enable-WebDAV-service)
+- [Installation](#installation)
+- [Initial configuration](#initial-configuration)
+- [Creating users](#creating-users)
+  - [Creating users with a Cloud Storage backend](#creating-users-with-a-cloud-storage-backend)
+  - [Creating users with a local encrypted backend (Data At Rest Encryption)](#creating-users-with-a-local-encrypted-backend-data-at-rest-Encryption)
+- [Virtual permissions](#virtual-permissions)
+- [Virtual folders](#virtual-folders)
+- [Configuration parameters](#configuration-parameters)
+  - [Use PostgreSQL data provider](#use-postgresql-data-provider)
+  - [Use MySQL/MariaDB data provider](#use-mysqlmariadb-data-provider)
+  - [Use CockroachDB data provider](#use-cockroachdb-data-provider)
+  - [Enable FTP service](#enable-ftp-service)
+  - [Enable WebDAV service](#enable-webdav-service)
 
 ## Installation
 
@@ -60,7 +60,7 @@ Let's create our first local user:
 
 ![Add user](./img/add-user.png)
 
-:warning: Please note that, on Linux, SFTPGo runs using a dedicated system user and group called `sftpgo`, for added security. If you want to be able to use directories outside `/srv/sftpgo` you need to set the appropriate system level permissions. For example if you define `/home/username/test` as home dir you have to create this directory yourself, if it doesn't exist, and set the appropriate system-level permissions:
+:warning: Please note that, on Linux, SFTPGo runs using a dedicated system user and group called `sftpgo`, for added security. If you want to be able to use directories outside the `/srv/sftpgo` path you need to set the appropriate system level permissions. For example if you define `/home/username/test` as home dir you have to create this directory yourself, if it doesn't exist, and set the appropriate system-level permissions:
 
 ```shell
 sudo mkdir /home/username/test
@@ -179,6 +179,9 @@ as you can see it worked as expected.
 
 ## Virtual folders
 
+A virtual folder is a mapping between a SFTPGo virtual path and a filesystem path outside the user home directory or on a different storage provider.
+Therefore, there is no need to create virtual folders for the users home directory or for directories within the users home directory.
+
 From the web admin interface click `Folders` and then the `+` icon.
 
 ![Add folder](./img/add-folder.png)

docs/howto/openssh-sftp-subsystem.md

@@ -3,6 +3,7 @@
 This tutorial shows how to run SFTPGo as OpenSSH's SFTP subsystem and still use its advanced features.
 
 Please note that when running in SFTP subsystem mode some SFTPGo features are not available, for example SFTPGo cannot limit the concurrent connnections or user sessions, restrict available ciphers etc. In this mode OpenSSH accepts the network connection, handles the SSH handshake and user authentication and then executes a separate SFTPGo process for each SFTP connection.
+Hooks may or may not work because OpenSSH stops the SFTPGo process when the user logs out and some hooks may not have run or ended yet. If you need these features use SFTPGo in standalone mode.
 
 ## Preliminary Note