buffer_list: add functions documentation

buffer_list: add functions documentation

bufferlist_* functions have no documentation whatsoever and the name is
not always enough to fully understand what the function is doing.
For this reason and for the sake of having better documented code, add
function doc in buffer.h.

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Steffan Karger <steffan@karger.me>
Message-Id: <20181010083731.31132-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17701.html
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit e72b2f2ce062c76c6ab658b7ae961f8b81cba307)