[PATCH proxmox-backup v2 13/13] docs: document garbage-collection maintenance mode

[Robert Obkircher <r.obkircher@proxmox.com>]

Signed-off-by: Robert Obkircher <r.obkircher@proxmox.com>
---
 docs/maintenance.rst | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/docs/maintenance.rst b/docs/maintenance.rst
index 46d9502d9..3459c097d 100644
--- a/docs/maintenance.rst
+++ b/docs/maintenance.rst
@@ -338,8 +338,8 @@ Refer to the :ref:`notifications` chapter for more details.
 Maintenance Mode
 ----------------
 
-Proxmox Backup Server supports setting `read-only` and `offline`
-maintenance modes on a datastore.
+Proxmox Backup Server supports setting `read-only`, `garbage-collection`, and
+`offline` maintenance modes on a datastore.
 
 Once enabled, depending on the mode, new reads and/or writes to the datastore
 are blocked, allowing an administrator to safely execute maintenance tasks, for
@@ -354,4 +354,13 @@ The supported maintenance modes are:
 
 - ``read-only``: Only read operations are allowed on the datastore.
 
+- ``garbage-collection``: Read operations and GC-related writes are allowed.
+
 - ``offline``: Neither read nor write operations are allowed on the datastore.
+
+The `garbage-collection` mode is designed to safely reclaim storage space when
+a datastore is nearly full. It allows prune jobs, garbage-collection, and
+snapshot deletions. Other writes, such as new backups, are blocked, so they
+cannot consume the remaining space. This is crucial for copy-on-write file
+systems like ZFS, where garbage-collection needs some additional space for
+metadata updates.
-- 
2.47.3