Documentation: clarify firmware_class provenance and why we can't rename the module

associating the device used to make the request as the device's parent.

The sysfs directory's file attributes are defined and controlled through

the new device's class (firmware_class) and group (fw_dev_attr_groups).

This is actually where the original firmware_class.c file name comes from,

as originally the only firmware loading mechanism available was the

mechanism we now use as a fallback mechanism.

This is actually where the original firmware_class module name came from,

given that originally the only firmware loading mechanism available was the

mechanism we now use as a fallback mechanism, which registers a struct class

firmware_class. Because the attributes exposed are part of the module name, the

module name firmware_class cannot be renamed in the future, to ensure backward

compatibility with old userspace.

To load firmware using the sysfs interface we expose a loading indicator,

and a file upload firmware into: