ftp: add explicit tls support

Add support for explicit FTP over TLS.

Fixes #4100

docs/content/ftp.md

@@ -61,6 +61,9 @@ password:
 Use FTP over TLS (Implicit)
 Enter a boolean value (true or false). Press Enter for the default ("false").
 tls> 
+Use FTP over TLS (Explicit)
+Enter a boolean value (true or false). Press Enter for the default ("false").
+explicit_tls> 
 Remote config
 --------------------
 [remote]
@@ -181,11 +184,29 @@ FTP password
 
 Use FTP over TLS (Implicit)
 
+When using implicit FTP over TLS the client will connect using TLS
+right from the start, which in turn breaks the compatibility with
+non-TLS-aware servers. This is usually served over port 990 rather
+than port 21. Cannot be used in combination with explicit FTP.
+
 - Config:      tls
 - Env Var:     RCLONE_FTP_TLS
 - Type:        bool
 - Default:     false
 
+#### --ftp-explicit-tls
+
+Use FTP over TLS (Explicit)
+
+When using explicit FTP over TLS the client explicitly request
+security from the server in order to upgrade a plain text connection
+to an encrypted one. Cannot be used in combination with implicit FTP.
+
+- Config:      explicit_tls
+- Env Var:     RCLONE_FTP_TLS
+- Type:        bool
+- Default:     false
+
 ### Advanced Options
 
 Here are the advanced options specific to ftp (FTP Connection).
@@ -243,6 +264,3 @@ FTP could support server side move but doesn't yet.
 
 Note that the ftp backend does not support the `ftp_proxy` environment
 variable yet.
-
-Note that while implicit FTP over TLS is supported,
-explicit FTP over TLS is not.