bpf: Copy comment about bpffs labelling into bpf_helpers.h

I found this comment in aosp/2103424 helpful in figuring out the correct
selinux domain for a newly added map.

Test: documentation only change
Change-Id: I96c06ff33605c8ed3f2ae6e8810bbcdc8e66b51b
1 file changed
tree: da67162898a0e64cb538e0ed92dbc16857f99d72
  1. staticlibs/