vsock: document namespace mode sysctls

vsock: document namespace mode sysctls

Add documentation for the vsock per-namespace sysctls (`ns_mode` and
`child_ns_mode`) to Documentation/admin-guide/sysctl/net.rst.
These sysctls were introduced by commit eafb64f40ca4 ("vsock: add
netns to vsock core").

Document the two namespace modes (`global` and `local`), the
inheritance behavior of `child_ns_mode`, and the restriction preventing
local namespaces from setting `child_ns_mode` to `global`.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://patch.msgid.link/20260216163147.236844-1-sgarzare@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>