Clarify when randomization happens for researchers: Fixes DOC-1182

@shnayder, see tracking_logs.rst: you mentioned this as a pain point for researchers.
@catong, your change (and another tweak) to create_video.rst
@mhoeber , fixed a typo in the glossary for content experiments
@WatsonEmily you asked to be tagged!
It's not necessarily when they first visit the component. It's the first time that data about any component connected to that experiment group is needed--this could e.g. happen when the student loads their progress page. The only guarantee is that it will happen before the student sees any of the tested content. 

To get precise data on who in each group actually saw the tested components, researchers should use the behavior events--problem checks, video views, etc.