Improve the _FILE_OFFSET_BITS docs and link to them.

We're still seeing lots of confusion. People do seem to look as far as
the headers, but stop there. So let's try a bit harder to point them in
the right direction.

Also explicitly state that removing _FILE_OFFSET_BITS=64 is the
behavior-preserving fix for compilation problems when upgrading to NDK
r15 or later.

Bug: N/A
Test: N/A
Change-Id: I2d5c65b2fb5cccb9977901e51fea1ad2ccc0fd95
diff --git a/libc/include/sys/sendfile.h b/libc/include/sys/sendfile.h
index ecdb76c..97432c0 100644
--- a/libc/include/sys/sendfile.h
+++ b/libc/include/sys/sendfile.h
@@ -34,6 +34,7 @@
 
 __BEGIN_DECLS
 
+/* See https://android.googlesource.com/platform/bionic/+/master/docs/32-bit-abi.md */
 #if defined(__USE_FILE_OFFSET64)
 ssize_t sendfile(int __out_fd, int __in_fd, off_t* __offset, size_t __count) __RENAME(sendfile64) __INTRODUCED_IN(21);
 #else