Improve comment for struct BufferDesc.
Add a note that per-buffer I/O condition variables currently live
outside the BufferDesc struct. Follow-up for commit
d8725104.
Reported-by: Julien Rouhaud <rjuju123@gmail.com>
Discussion: https://postgr.es/m/
20210311031118.hucytmrgwlktjxgq%40nol