Document the group API in pmc(3): pmc_allocate_group, pmc_group_create,
pmc_group_add, pmc_group_commit, pmc_group_read, and pmc_read_pair.
Explain what a group guarantees, when a commit fails with ENOSPC, and
how to scale a multiplexed count from the enabled and running times.
Document the -b flag and the brace-list syntax in pmcstat(8), including
how a modifier inside the list differs from one after it, and what the
residency column means.
Document the PMCGROUPINHERITMISS record in pmclog(3).
Sponsored by: AMD
Signed-off-by: Andre Silva <andasilv@amd.com>