### What does this PR do?
Update FFI documentation with a note on Windows API handle values, as
pointer encoding to double causes intermittent failures with some
classes of handles.
Putting it in this accordion feels less than ideal but it's also a very
specific use case.
The specific issue I experienced: HDCs and HBITMAPs are basically 32
bits, although they are typedef'd in the headers to HANDLE. They are
returned from and passed to the GDI APIs as sign-extended versions of
the underlying 32-bit value, and so when going through the ptr <->
double pathway of bun FFI, the bottom 11 bits of those values are lost
if the original value had bit 31 set, and subsequent calls will fail.
Probably this is fixable by correctly encoding 'negative' pointers in
the double representation, and I might tackle that if I find time.
1879b7eeca