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