bpf, docs: Add note about type convention

Add explanation about use of "u64", "u32", etc. as
the type convention used in BPF documentation.

Signed-off-by: Dave Thaler <dthaler@microsoft.com>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230127014706.1005-1-dthaler1968@googlemail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>