docs: bpf: Clarify -mcpu=v3 requirement for atomic ops

Alexei pointed out [1] that this wording is pretty confusing. Here's
an attempt to be more explicit and clear.

[1] https://lore.kernel.org/bpf/CAADnVQJVvwoZsE1K+6qRxzF7+6CvZNzygnoBW9tZNWJELk5c=Q@mail.gmail.com/



Signed-off-by: Brendan Jackman <jackmanb@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210120133946.2107897-3-jackmanb@google.com