Documentation: networking: Add a caveat to nexthop_compat_mode sysctl

net.ipv4.nexthop_compat_mode was added when nexthop objects were added to
provide the view of nexthop objects through the usual lens of the route
UAPI. As nexthop objects evolved, the information provided through this
lens became incomplete. For example, details of resilient nexthop groups
are obviously omitted.

Now that 16-bit nexthop group weights are a thing, the 8-bit UAPI cannot
convey the >8-bit weight accurately. Instead of inventing workarounds for
an obsolete interface, just document the expectations of inaccuracy.

Fixes: b72a6a7 ("net: nexthop: Increase weight to u16")
Signed-off-by: Petr Machata <[email protected]>
Reviewed-by: Ido Schimmel <[email protected]>
Reviewed-by: David Ahern <[email protected]>
Link: https://patch.msgid.link/b575e32399ccacd09079b2a218255164535123bd.1733740749.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <[email protected]>

Documentation/networking/ip-sysctl.rst

@@ -2170,6 +2170,12 @@ nexthop_compat_mode - BOOLEAN
 	understands the new API, this sysctl can be disabled to achieve full
 	performance benefits of the new API by disabling the nexthop expansion
 	and extraneous notifications.
+
+	Note that as a backward-compatible mode, dumping of modern features
+	might be incomplete or wrong. For example, resilient groups will not be
+	shown as such, but rather as just a list of next hops. Also weights that
+	do not fit into 8 bits will show incorrectly.
+
 	Default: true (backward compat mode)
 
 fib_notify_on_flag_change - INTEGER