Fix documentation of equality functions. (#84847)

The first line of the documentation of equality functions listEquals, mapEquals, and setEquals claimed that they are deep equality functions. A later paragraph explained that they check for deep equality only if the elements, which are collections, implement the equality operator to do so. That is almost never the case.

The first line of the documentation was changed to element-by-element equality, and a reference was added to the DeepCollectionEquality class.