seg6: man: ip-link.8: add SRv6 End PSP flavor description

seg6: man: ip-link.8: add SRv6 End PSP flavor description

This patch extends the manpage by providing a brief description of the PSP
flavor for the SRv6 End 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 flavors psp dev eth0

Standard Output:
ip -6 route show 2001:db8::1
2001:db8::1  encap seg6local action End flavors psp 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",
	"flavors": [ "psp" ],
	"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: David Ahern <dsahern@kernel.org>