seg6: man: ip-link.8: add description of NEXT-C-SID flavor for SRv6 End.X behavior

seg6: man: ip-link.8: add description of NEXT-C-SID flavor for SRv6 End.X behavior

This patch extends the manpage by providing the description of NEXT-C-SID
support for the SRv6 End.X behavior as defined in RFC 8986 [1].

The code/logic required to handle the "flavors" framework has already been
merged into iproute2 by commit:
    04a6b456bf74 ("seg6: add support for flavors in SRv6 End* behaviors").

Some examples:
ip -6 route add 2001:db8::1 encap seg6local action End.X nh6 fc00::1 flavors next-csid dev eth0

Standard Output:
ip -6 route show 2001:db8::1
2001:db8::1  encap seg6local action End.X nh6 fc00::1 flavors next-csid lblen 32 nflen 16 dev eth0 metric 1024 pref medium

JSON Output:
ip -6 -j -p route show 2001:db8::1
[ {
	"dst": "2001:db8::1",
	"encap": "seg6local",
        "action": "End.X",
        "nh6": "fc00::1",
        "flavors": [ "next-csid" ],
        "lblen": 32,
        "nflen": 16,
	"dev": "eth0",
	"metric": 1024,
	"flags": [ ],
	"pref": "medium"
} ]

[1] - https://datatracker.ietf.org/doc/html/rfc8986

Signed-off-by: Paolo Lungaroni <paolo.lungaroni@uniroma2.it>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>