gpiolib: make struct comments into real kernel docs

[ Upstream commit 4398693a ]

We have several comments that start with '/**' but don't conform to the
kernel doc standard. Add proper detailed descriptions for the affected
definitions and move the docs from the forward declarations to the
struct definitions where applicable.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Stable-dep-of: bdbbae24 ("gpiolib: protect the GPIO device against being dropped while in use by user-space")
Signed-off-by: Sasha Levin <sashal@kernel.org>