docs/licensing: Clarify wording about "GPL" and "Proprietary"

docs/licensing: Clarify wording about "GPL" and "Proprietary"

There are currently some doubts about out-of-tree kernel modules licensed
under GPLv3 and if they are supposed to be able to use symbols exported
using EXPORT_SYMBOL_GPL.

Clarify that "Proprietary" means anything non-GPL2 even though the
license might be an open source license. Also disambiguate "GPL
compatible" to "GPLv2 compatible".

Signed-off-by: Uwe Kleine-König <[email protected]>
Signed-off-by: Jonathan Corbet <[email protected]>
Link: https://lore.kernel.org/r/[email protected]