[patch] Improve generated libstdc++ API docs

[Jonathan Wakely]

This adjusts some Doxygen comments and updates the Doxygen config file
to ensure all headers are processed (previously doxygen was ignoring
filenames without an extension, which is a lot of the library!)

There's a workaround in include/std/bitset for the bug 
https://bugzilla.gnome.org/show_bug.cgi?id=759242
and another in include/ext/pb_ds/detail/bin_search_tree_/traits.hpp
for https://bugzilla.gnome.org/show_bug.cgi?id=759241

Tested with 'make doc-man-doxygen doc-html-doxygen' and committed to
trunk.


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