net: doc: add default value for neighbour parameters

	Maximum number of neighbor entries allowed.  Increase this

	when using large numbers of interfaces and when communicating

	with large numbers of directly-connected peers.

	Default: 1024

neigh/default/unres_qlen_bytes - INTEGER

	The maximum number of bytes which may be used by packets

	queued for each	unresolved address by other network layers.

	(added in linux 3.3)

	Seting negative value is meaningless and will retrun error.

	Default: 65536 Bytes(64KB)

neigh/default/unres_qlen - INTEGER

	The maximum number of packets which may be queued for each

	unresolved address by other network layers.

	(deprecated in linux 3.3) : use unres_qlen_bytes instead.

	Prior to linux 3.3, the default value is 3 which may cause

	secluded packet loss. The current default value is calculated

	according to default value of unres_qlen_bytes and true size of

	packet.

	Default: 31

mtu_expires - INTEGER

	Time, in seconds, that cached PMTU information is kept.