doc: use @name to define a group, not @group

doc: use @name to define a group, not @group

we are able to output a specified group using the directive
of `doxygengroup` in breathe. this directive prints out the
description of the group. but it's not realistic to enumerate
all groups defined in source code in the rst files. but the
doxygen command @name also helps to group functions together.
the downside of this approach is that we can not add more
items to a group later on. but it should be fine with us,
since in our case, all the grouped items are living in a single
header file.

Fixes: #9442
Signed-off-by: Kefu Chai <kchai@redhat.com>