libstdc++: Use ///< for inline documentation

libstdc++: Use ///< for inline documentation

I noticed that some variables were misdocumented when using trailing
comment for documentation.  I ran a search with a relatively simple
regex[1] to look for any ///s following some code that did not have a <,
and came up with these instances only.

[1]: \s*([^  ]+\s*)+///[^<].*$

libstdc++-v3/ChangeLog:

	* include/std/iostream: Use ///< for inline documentation.
	* include/std/limits: Likewise.
	* include/experimental/internet: Likewise.