docs: net: document neigh gc_stale_time sysctl

Add missing documentation for a neighbor table garbage collector sysctl
parameter in ip-sysctl.rst:

neigh/default/gc_stale_time: controls how long an unused neighbor entry
is kept before becoming eligible for garbage collection (default: 60
seconds)

Signed-off-by: Gabriel Goller <g.goller@proxmox.com>
Link: https://patch.msgid.link/20260223101257.47563-1-g.goller@proxmox.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>