bpf, doc: Add heading and example for extensions in cbpf

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@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/CAADnVQJ=DoRDcVkaXmY3EmNdLoO7gq1mkJOn5G=00wKH8qUtZQ@mail.gmail.com

Documentation/networking/filter.rst

@@ -320,13 +320,6 @@ Examples for low-level BPF:
   ret #-1
   drop: ret #0
 
-**(Accelerated) VLAN w/ id 10**::
-
-  ld vlan_tci
-  jneq #10, drop
-  ret #-1
-  drop: ret #0
-
 **icmp random packet sampling, 1 in 4**::
 
   ldh [12]
@@ -358,6 +351,22 @@ Examples for low-level BPF:
   bad: ret #0             /* SECCOMP_RET_KILL_THREAD */
   good: ret #0x7fff0000   /* SECCOMP_RET_ALLOW */
 
+Examples for low-level BPF extension:
+
+**Packet for interface index 13**::
+
+  ld ifidx
+  jneq #13, drop
+  ret #-1
+  drop: ret #0
+
+**(Accelerated) VLAN w/ id 10**::
+
+  ld vlan_tci
+  jneq #10, drop
+  ret #-1
+  drop: ret #0
+
 The above example code can be placed into a file (here called "foo"), and
 then be passed to the bpf_asm tool for generating opcodes, output that xt_bpf
 and cls_bpf understands and can directly be loaded with. Example with above