fix line joints when doing a PI angle

fixes #1037
doc: documented parameters to 'on_selection_change' event in adapters/listadapter.py
doc: removed redundant event description

Removed redundant event description as link goes fo fully documented
event description.
doc: corrected 'on_selection_changed' description based on real parameters
doc: added link to uix/listview.py
doc: revisions to adapters/__init__.py
doc: reinstated mvc reference, add view to adapters/__init__.py
Added pong note about ball bouncing
video: fix video.unload called when position is changing. Closes #2275
config: ensure python3 configparser will always set strings. Closes #2122
example: add slider volume for audio examples. Refs #1869
Merge pull request #2421 from kivy/fix_line_joints

Fix line joints when doing a PI angle
doc: Fixed spelling of highlight