man: document tunnel options in ip-route.8.in

man: document tunnel options in ip-route.8.in

Add missing tunnel options [GENEVE_OPTS | VXLAN_OPTS | ERSPAN_OPTS] and
their descriptions to ip-route.8.in.

Additionally, document each parameter of the ip ENCAPHDR section, aligning
it with the style used for other ENCAPHDR descriptions. Most of the
content is adapted from tc-tunnel_key.8 for consistency.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>