doc: services: disk_access: add documentation for INIT/DEINIT IOCTLs

The disk access API provides access to storage devices.

Initializing Disks

******************

Since many disk devices (such as SD cards) are hotpluggable, the disk access

API provides IOCTLs to initialize and de-initialize the disk. They are

as follows:

* :c:macro:`DISK_IOCTL_CTRL_INIT`: Initialize the disk. Must be called before

  additional I/O operations can be run on the disk device. Equivalent to

  calling the legacy function :c:func:`disk_access_init`.

* :c:macro:`DISK_IOCTL_CTRL_DEINIT`: De-initialize the disk. Once this IOCTL

  is issued, the :c:macro:`DISK_IOCTL_CTRL_INIT` must be issued before

  the disk can be used for addition I/O operations.

Init/deinit IOCTL calls are balanced, so a disk will not de-initialize until

an equal number of deinit IOCTLs have been issued as init IOCTLs.

It is also possible to force a disk de-initialization by passing a

pointer to a boolean set to ``true`` as a parameter to the

:c:macro:`DISK_IOCTL_CTRL_DEINIT` IOCTL. This is an unsafe operation which

each disk driver may handle differently, but it will always return

a value indicating success.

Note that de-initializing a disk is a low level operation- typically the

de-initialization and initialization calls should be left to the filesystem

implementation, and the user application should not need to manually

de-initialize the disk and can instead call :c:func:`fs_unmount`

SD Card support

***************

can access the SD cards via disk access API.

Both standard and high-capacity SD cards are supported.

.. note:: The system does not support inserting or removing cards while the

   system is running. The cards must be present at boot and must not be

   removed. This may be fixed in future releases.

   FAT filesystems are not power safe so the filesystem may become

   corrupted if power is lost or if the card is removed.

.. note:: FAT filesystems are not power safe so the filesystem may become

   corrupted if power is lost or if the card is removed without unmounting

   the filesystem

SD Memory Card subsystem

========================