docs: document 'dirs' parameter for directory storage

Signed-off-by: Leo Nunner <l.nunner@proxmox.com>

pve-storage-cifs.adoc

@@ -57,6 +57,10 @@ path::
 
 The local mount point. Optional, defaults to `/mnt/pve/<STORAGE_ID>/`.
 
+dirs::
+
+Overrides for the default directory layout. Optional.
+
 .Configuration Example (`/etc/pve/storage.cfg`)
 ----
 cifs: backup

pve-storage-dir.adoc

@@ -46,9 +46,18 @@ storage backends.
 Configuration
 ~~~~~~~~~~~~~
 
-This backend supports all common storage properties, and adds an
-additional property called `path` to specify the directory. This
-needs to be an absolute file system path.
+This backend supports all common storage properties, and adds two
+additional properties. The `path` property is used to specify the
+directory. This needs to be an absolute file system path.
+
+The optional `dirs` property allows for the default layout to be
+changed. It consists of a comma-separated list of identifiers in
+the following format:
+
+ vtype:path
+
+Where `vtype` is one of the allowed content types for the storage, and
+`path` is an absolute file system path.
 
 .Configuration Example (`/etc/pve/storage.cfg`)
 ----
@@ -57,12 +66,12 @@ dir: backup
         content backup
         prune-backups keep-last=7
         max-protected-backups 3
+        dirs backup:/custom/backup/dir
 ----
 
 The above configuration defines a storage pool called `backup`. That pool can be
 used to store up to 7 regular backups (`keep-last=7`) and 3 protected backups
-per VM. The real path for the backup files is `/mnt/backup/dump/...`.
-
+per VM. The real path for the backup files is `/mnt/backup/custom/backup/dir/...`.
 
 File naming conventions
 ~~~~~~~~~~~~~~~~~~~~~~~

pve-storage-nfs.adoc

@@ -40,6 +40,10 @@ path::
 
 The local mount point (defaults to `/mnt/pve/<STORAGE_ID>/`).
 
+dirs::
+
+Overrides for the default directory layout. Optional.
+
 options::
 
 NFS mount options (see `man nfs`).