Fix grouping for latest doxygen.

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.

Signed-off-by: Ralph Giles <giles@thaumas.net>
1 job for doxygen in 45 seconds (queued for 5 seconds)