staging: iio: documentation rewrite and cleanup of sysfs documetation

Change to capital syntax for documetation e.g. 'inX-inY_raw'
Use multiple 'What:' entries rather than wild cards or options so
as to make the documentation clearer and easier to search.
Some trivial spelling fixes.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>