net/core: Document reuseport_add_sock() bind_inany argument

This patch avoids that the following warning is reported when building
with W=1:

warning: Function parameter or member 'bind_inany' not described in 'reuseport_add_sock'

Cc: Martin KaFai Lau <kafai@fb.com>
Fixes: 2dbb9b9e ("bpf: Introduce BPF_PROG_TYPE_SK_REUSEPORT") # v4.19.
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: David S. Miller <davem@davemloft.net>