lib: annotate versioned functions

Every implementation of a particular version of given symbol needs to be
marked in its declaration as such (using `__vsym` macro).  This patch
fixes this and also clarifies the documentation about that.

Signed-off-by: Andrzej Ostruszka <aostruszka@marvell.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>