docs: bpf: Clarify BPF_CMPXCHG wording

Based on [1], BPF_CMPXCHG should always load the old value into R0. The
phrasing in bpf.rst is somewhat ambiguous in this regard, improve it to
make this aspect crystal clear.

  [1] https://lore.kernel.org/bpf/CAADnVQJFcFwxEz=wnV=hkie-EDwa8s5JGbBQeFt1TGux1OihJw@mail.gmail.com/

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210210142853.82203-1-iii@linux.ibm.com

Documentation/networking/filter.rst

@@ -1076,8 +1076,8 @@ off``. ::
     BPF_CMPXCHG
 
 This atomically compares the value addressed by ``dst_reg + off`` with
-``R0``. If they match it is replaced with ``src_reg``, The value that was there
-before is loaded back to ``R0``.
+``R0``. If they match it is replaced with ``src_reg``. In either case, the
+value that was there before is zero-extended and loaded back to ``R0``.
 
 Note that 1 and 2 byte atomic operations are not supported.