[doc, prim] doc update for new prim library
[doc, prim] doc update for new prim library

Signed-off-by: Joshua Park <df42862f34d3e72c8680ad6848ac1dfc28d3f0ab@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan