libstdc++: Reduce duplication in Doxygen comments for std::list

libstdc++: Reduce duplication in Doxygen comments for std::list

We have a number of comments which are duplicated for C++98 and C++11
overloads, where the signatures are slightly different. Instead of
duplicating the comments that are 90% identical, just use a single
comment that can apply to both. In some cases this means saying "an
iterator" instead of "A const iterator" but that's fine, a
std::list::const_iterator is still an iterator (and a non-const iterator
is a valid argument to those functions because they'll implicitly
convert to const_iterator).

In two cases the @return description just needs to say that it returns
void for C++98 and an iterator otherwise.

libstdc++-v3/ChangeLog:

	* include/bits/stl_list.h: Reduce duplication in doxygen
	comments.