[PATCH v2 bpf-nxt] Documentation/bpf: Add heading and example for extensions in filter.rst

[John Fastabend]

Roy, UjjaL wrote:
> [1] https://www.kernel.org/doc/html/latest/bpf/
> 
> Add new heading for extensions to make it more readable. Also, add one
> more example of filtering interface index for better understanding.
> 
> Signed-off-by: Roy, UjjaL <royujjal at gmail.com>
> Acked-by: Song Liu <songliubraving at fb.com>

Looks OK to me. I thought the original was readable without the header, but
if it helps someone seems easy enough to do.

Acked-by: John Fastabend <john.fastabend at gmail.com>