Mention the POSIX header tests in docs/status.md.

Also add a couple of comments in the tests for ease of understanding
when grepping for `__BIONIC__`.

Bug: N/A
Test: N/A
Change-Id: I7833a3ffbcc3badf9cec95f268d11a6d8a5ff9aa
diff --git a/tests/headers/posix/netinet_in_h.c b/tests/headers/posix/netinet_in_h.c
index 6e0be1c..cd89685 100644
--- a/tests/headers/posix/netinet_in_h.c
+++ b/tests/headers/posix/netinet_in_h.c
@@ -62,7 +62,7 @@
 
   TYPE(struct ipv6_mreq);
   STRUCT_MEMBER(struct ipv6_mreq, struct in6_addr, ipv6mr_multiaddr);
-#if defined(__BIONIC__)
+#if defined(__BIONIC__) // Currently comes from uapi header.
   STRUCT_MEMBER(struct ipv6_mreq, int, ipv6mr_interface);
 #else
   STRUCT_MEMBER(struct ipv6_mreq, unsigned, ipv6mr_interface);