bpf, docs: Clarify that MOVSX is only for BPF_X not BPF_K

Per discussion on the mailing list at
https://mailarchive.ietf.org/arch/msg/bpf/uQiqhURdtxV_ZQOTgjCdm-seh74/
the MOVSX operation is only defined to support register extension.

The document didn't previously state this and incorrectly implied
that one could use an immediate value.

Signed-off-by: Dave Thaler <dthaler1968@gmail.com>
Acked-by: David Vernet <void@manifault.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240118232954.27206-1-dthaler1968@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>