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!
"known as A/B or split test*s*"? (missing s)