ion: minor clean up

-- init rb nodes in ion_handle_create
-- in ion_handle_destroy, check that a node belongs to a tree before removing
   it (safety check, does not happen right now)
-- mark as static functions used only inside ion.c
-- update comments to ion_share() with a relevant blurb from the implementation
-- other minor updates/typo fixes to comments

Signed-off-by: Iliyan Malchev <malchev@google.com>