improved affordance definition and def source

[cameronmore]

Improved the definition slightly and added a proper source

[mark-jensen]

I think "used by" is too strong. First it is restricted to material entities, but the def refers to ICs. Second, it seems to me that in some cases the IC can provide an affordance without being "used" by the agent. Say, eg., a site filled with water momentarily drains so that an agent can count how many starfish are located there. I don't think the agent is using the site, but it is plausible that some RE of that site is aiding the agent to accomplish some task. Weak, yes, but better examples probably exist.

Please undo the replacement of 'creates an opportunity for'. I realize this is still not great, but I think some thought went into it back in the day. The other changes are good

[cameronmore]

@mark-jensen that makes sense, the change has been made