DOC: config: mention that idle connection sharing is per thread-group

DOC: config: mention that idle connection sharing is per thread-group

There's already a tunable "tune.idle-pool.shared" allowing to enable or
disable idle connection sharing between threads. However the doc does not
mention that these connections are only shared between threads of the same
thread group, since 2.7 with commit 15c5500b6e ("MEDIUM: conn: make
conn_backend_get always scan the same group"). Let's clarify this and
also give a hint about "max-threads-per-group" which can be helpful for
machines with unified caches.