Update README.tuplock
Multixact truncation is now handled differently, and this file hadn't
gotten the memo.
Per note from Amit Langote. I didn't use his patch, though.
Also update the description of infomask bits, which weren't completely up
to date either. This commit also propagates
b01a4f6838 back to 9.3 and
9.4, which apparently I failed to do back then.