Fix grouping for latest doxygen.

Open Alexander Traud requested to merge traud/vorbis:traud-master-patch-73993 into master

Since doxygen 1.8.16, opening and closing a group must not be done as C comment but as doxygen command. In other words, not one but two asterisk characters are required so that doxygen finds a group. Found because of opusfile#2332 (closed). Finally, a line break within the text/title of a reference is not supported and broke the link text.

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