Documentation: Document the kernel flag bdev_allow_write_mounted

Commit ed5cc702d311 ("block: Add config option to not allow writing to mounted
devices") added a Kconfig option along with a kernel command-line tuning to
control writes to mounted block devices, as a means to deal with fuzzers like
Syzkaller, that provokes kernel crashes by directly writing on block devices
bypassing the filesystem (so the FS has no awareness and cannot cope with that).

The patch just missed adding such kernel command-line option to the kernel
documentation, so let's fix that.

Cc: Bart Van Assche <[e-mail]>
Cc: Darrick J. Wong <[e-mail]>
Cc: Jens Axboe <[e-mail]>
Reviewed-by: Jan Kara <[e-mail]>
Signed-off-by: Guilherme G. Piccoli <[e-mail]>
Reviewed-by: Darrick J. Wong <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/admin-guide/kernel-parameters.txt

···
 			Format: <io>,<irq>,<mode>
 			See header of drivers/net/hamradio/baycom_ser_hdx.c.
 
+	bdev_allow_write_mounted=
+			Format: <bool>
+			Control the ability to open a mounted block device
+			for writing, i.e., allow / disallow writes that bypass
+			the FS. This was implemented as a means to prevent
+			fuzzers from crashing the kernel by overwriting the
+			metadata underneath a mounted FS without its awareness.
+			This also prevents destructive formatting of mounted
+			filesystems by naive storage tooling that don't use
+			O_EXCL. Default is Y and can be changed through the
+			Kconfig option CONFIG_BLK_DEV_WRITE_MOUNTED.
+
 	bert_disable	[ACPI]
 			Disable BERT OS support on buggy BIOSes.