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 <amir73il@gmail.com>
Signed-off-by: Chen Linxuan <chenlinxuan@uniontech.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>