[patch] Improve some doxygen comments

[Jonathan Wakely]

One last set of Doxygen-related updates for today.

This removes some redundancy, moves std::bitset to the "Utilities"
group in the docs (which is where it lives in the C++11 standard) and
gets rid of the "Global I/O operators for bitsets" group, which only
contained two operators for the dytnamic_bitset extension.

Tested x86_64-linux, committed to trunk.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: patch.txt
Type: text/x-patch
Size: 8593 bytes
Desc: not available
URL: <http://gcc.gnu.org/pipermail/libstdc++/attachments/20140614/28455f7e/attachment.bin>