devres: Clarify documentation for devres_destroy()

It's not massively obvious (at least to me) that removing and freeing a
resource does not involve calling the release function for the resource
but rather only removes the management of it. Make the documentation more
explicit.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>