doc: fuse: Add max_background and congestion_threshold

As I preparing patches adding selftests for fusectl,
I notice that documentation of max_background and congestion_threshold
is missing.

This patch add some descriptions about these two files.

Cc: Amir Goldstein <[e-mail]>
Signed-off-by: Chen Linxuan <[e-mail]>
Signed-off-by: Miklos Szeredi <[e-mail]>

Documentation/filesystems/fuse.rst

···
 	  connection.  This means that all waiting requests will be aborted an
 	  error returned for all aborted and new requests.
 
+        max_background
+          The maximum number of background requests that can be outstanding
+          at a time. When the number of background requests reaches this limit,
+          further requests will be blocked until some are completed, potentially
+          causing I/O operations to stall.
+
+        congestion_threshold
+          The threshold of background requests at which the kernel considers
+          the filesystem to be congested. When the number of background requests
+          exceeds this value, the kernel will skip asynchronous readahead
+          operations, reducing read-ahead optimizations but preserving essential
+          I/O, as well as suspending non-synchronous writeback operations
+          (WB_SYNC_NONE), delaying page cache flushing to the filesystem.
+
 Only the owner of the mount may read or write these files.
 
 Interrupting filesystem operations