docs: net: fix minor issues with checksum offloads

docs: net: fix minor issues with checksum offloads

Update the checksum offload documentation to match current code:

- SCTP CRC32c offload requires NETIF_F_SCTP_CRC, not ordinary IP
  checksum offload
- NETIF_F_IP_CSUM and NETIF_F_IPV6_CSUM are restricted legacy
  features; new devices should use NETIF_F_HW_CSUM
- GRE LCO is handled by the shared gre_build_header() helper used by
  both IPv4 and IPv6 GRE
- VXLAN_F_REMCSUM_TX is a VXLAN configuration flag, not a field of
  struct vxlan_rdst

Link: https://patch.msgid.link/20260526160151.2793354-8-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>