Document the debugfs API

This is an updated document covering the internal API for the debugfs
filesystem.  Thanks to Shen Feng for suggesting that I put this text here
and noting that the old LWN version was rather out of date.

Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Reported-by: Shen Feng <shen@cn.fujitsu.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>