DOC: add missing parameters doc about QGis plugin

Information comes from https://docs.qgis.org/testing/en/docs/user_manual/processing/3rdParty.html#documentation-of-otb-settings-available-in-qgis-processing

requested review from @thibrom

assigned to @tristanlaurent

mentioned in merge request !1011 (merged)

mentioned in commit 6fbacb3f

merged

added documentation label

changed milestone to %9.1.0