docs/pcloud: document the cleanup issues

docs/content/pcloud.md

@@ -112,6 +112,13 @@ Deleted files will be moved to the trash.  Your subscription level
 will determine how long items stay in the trash.  `rclone cleanup` can
 be used to empty the trash.
 
+### Emptying the trash
+
+Due to an API limitation, the `rclone cleanup` command will only work if you 
+set your username and password in the advanced options for this backend. 
+Since we generally want to avoid storing user passwords in the rclone config
+file, we advise you to only set this up if you need the `rclone cleanup` command to work.
+
 ### Root folder ID
 
 You can set the `root_folder_id` for rclone.  This is the directory
@@ -251,4 +258,32 @@ Properties:
     - "eapi.pcloud.com"
         - EU region
 
+#### --pcloud-username
+
+Your pcloud username.
+			
+This is only required when you want to use the cleanup command. Due to a bug
+in the pcloud API the required API does not support OAuth authentication so
+we have to rely on user password authentication for it.
+
+Properties:
+
+- Config:      username
+- Env Var:     RCLONE_PCLOUD_USERNAME
+- Type:        string
+- Required:    false
+
+#### --pcloud-password
+
+Your pcloud password.
+
+**NB** Input to this must be obscured - see [rclone obscure](/commands/rclone_obscure/).
+
+Properties:
+
+- Config:      password
+- Env Var:     RCLONE_PCLOUD_PASSWORD
+- Type:        string
+- Required:    false
+
 {{< rem autogenerated options stop >}}