Changes

distances: clarify the documentation of matrices vs distances
Functions manipulate distance matrix structures, so don't just talk 
about "distances" so that people aren't confused with individual
distance values inside those matrices.
Thanks to Thomas Gruber for the report.
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>
(cherry picked from commit a1b0f2c24704b8f0f02437854382264f14a94f09)