CSS: refresh language about old IE

[timmywil]

currentStyle and runtimeStyle are only in old IE, but the docs just say "Internet Explorer." We probably want to specify that it only applies to IE<9.

See jquery/jquery#2837

[mgol]

Should we even mention IE<9 here? We're dropping support for it so it doesn't make much sense to advertise what we used to do for it.

[dmethvin]

Once IE8 is really dead I can see dropping it, but it's useful info for people who still have to deal with it. Maybe in a year or two we make a run through the docs and remove all the IE apologies. 😺