[optional.inplace] Remove redundant explanation of in_place_t.

The existing wording in this paragraph is already normatively covered
by [optional.object.ctor], so refer there. This also removes the
term "argument pack" that isn't defined by the standard.

Also shorten "constructor and function overloading", since in_place_t
is in fact not used to disambiguate anything but constructors.

Thanks to Alisdair Meredith for pointing out the incorrect term
"argument pack".