lib/pipeline: add a missing docunit

Signed-off-by: Jean Privat <jean@pryen.org>
lib: remove warnings on hash_debug, more_collections, ordered_tree, and poset

Signed-off-by: Jean Privat <jean@pryen.org>
lib/more_collections: improve doc and docunits

Signed-off-by: Jean Privat <jean@pryen.org>
lib/ordered_tree: improve doc and docunits

Signed-off-by: Jean Privat <jean@pryen.org>
lib/poset: add POSet::add_chain

Signed-off-by: Jean Privat <jean@pryen.org>
lib/poset: improve doc and docunits

Signed-off-by: Jean Privat <jean@pryen.org>