Add documentation to MultiBody.
Add documentation to MultiBody.

jrl-umi3218/RBDyn,jrl-umi3218/RBDyn,jrl-umi3218/RBDyn,gergondet/RBDyn,gergondet/RBDyn,jrl-umi3218/RBDyn,gergondet/RBDyn,gergondet/RBDyn,gergondet/RBDyn