Documentation Peer Instruction

[thiemehennis]

it's not clear from the instruction on page

https://github.com/edx/edx-documentation/blob/master/en_us/shared/exercises_tools/peer_instruction_tool.rst

that Peer Instruction, if using the Open Edx version (not the partner edX version), needs to be installed as X-Block before being able to enable it in Settings/Adv. Settings. Could be improved in the documentation.