libstdc++: Add Doxygen comment for string::resize_and_overwite

This is a complicated API that should be clearly documented.

Also improve the comment on basic_ios::_M_setstate.

libstdc++-v3/ChangeLog:

	* include/bits/basic_ios.h (basic_ios::_M_setstate): Add
	caveat to comment.
	* include/bits/basic_string.h (resize_and_overwrite): Add
	doxygen comment.