Summary:
Adds missing @param docs and fixes an incorrectly documented one.
Clarifies difference between 'length' and 'position', specifying the
latter is a number between 0.0 and 1.0. Improves verbage here and there
for grammatical correctness and internal consistency.
Subscribers: cedric
Differential Revision: https://phab.enlightenment.org/D5780
Signed-off-by: Cedric Bail <cedric@osg.samsung.com>