doc: Document new environment variabless to specify secret values

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

docs/backup-client.rst

@@ -49,15 +49,48 @@ Environment Variables
   When set, this value is used for the password required for the backup server.
   You can also set this to a API token secret.
 
+  .. Note:: if not set, we also query ``PBS_PASSWORD_FD``,
+     ``PBS_PASSWORD_FILE`` and ``PBS_PASSWORD_CMD`` (in that order)
+     and return the first value set.
+
+``PBS_PASSWORD_FD``
+  Like ``PBS_PASSWORD``, but read data from Unix file descriptor.
+
+``PBS_PASSWORD_FILE``
+  Like ``PBS_PASSWORD``, but read data from specified file name.
+
+``PBS_PASSWORD_CMD``
+  Like ``PBS_PASSWORD``, but read data from specified command (stdout).
+
 ``PBS_ENCRYPTION_PASSWORD``
   When set, this value is used to access the secret encryption key (if
   protected by password).
 
+  .. Note:: if not set, we also query ``PBS_ENCRYPTION_PASSWORD_FD``,
+     ``PBS_ENCRYPTION_PASSWORD_FILE`` and
+     ``PBS_ENCRYPTION_PASSWORD_CMD`` (in that order) and return the
+     first value set.
+
+``PBS_ENCRYPTION_PASSWORD_FD``
+  Like ``PBS_ENCRYPTION_PASSWORD``, but read data from Unix file descriptor.
+
+``PBS_ENCRYPTION_PASSWORD_FILE``
+  Like ``PBS_ENCRYPTION_PASSWORD``, but read data from specified file name.
+
+``PBS_ENCRYPTION_PASSWORD_CMD``
+  Like ``PBS_ENCRYPTION_PASSWORD``, but read data from specified command (stdout).
+
 ``PBS_FINGERPRINT`` When set, this value is used to verify the server
   certificate (only used if the system CA certificates cannot validate the
   certificate).
 
 
+.. Note:: Passwords must be valid UTF8 an may not contain
+   newlines. For your convienience, we just use the first line as
+   password, so you can add arbitrary comments after the
+   first newline.
+
+
 Output Format
 -------------