thread: document undocumented classes
thread: document undocumented classes

thread_scheduling group and thread_attributes are undocumented. The latter
is marked as internal but it shouldn't be, since it is clearly used by outside
callers.

Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>
Message-Id: <1461680406-14500-2-git-send-email-7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

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