doc: tuning: add note about 64-bit OpenSSL speedup

doc: tuning: add note about 64-bit OpenSSL speedup

Since switching HTTPS termination to 64-bit, I've noticed a
significant CPU usage reduction on public-inbox.org.  IMAPS,
NNTPS, and POP3S remain 32-bit for the moment since that doesn't
go through varnish and I haven't gotten around to dealing with
a 64-bit Xapian (or SQLite) install.

My current HTTPS setup is:

  64-bit PublicInbox::PsgiRproxy -> varnish -> 32-bit PublicInbox::WWW

IMAPS, NNTPS, POP3S hit the same 32-bit public-inbox-netd
process directly without a caching layer.