KEY UPDATE / UPDATE / NO KEY UPDATE distinction vs. README.tuplock

Hi,

It seems that README.tuplock never got updated when the KEY SHARE patch's
lock level were changed from being KEY UPDATE / UPDATE / SHARE / KEY SHARE
to UPDATE / NO KEY UPDATE / SHARE / KEY SHARE.

Thus, as it stands, that file implies that SELECT FOR UPDATE obtains a
weaker lock than an actual UPDATE might take (if that update modifies key
columns) -- according to README.tuplock, the former doesn't conflict with
KEY SHARE while the actual UPDATE would.

But this isn't actually the case in the committed version of the patch -
one now needs to explicitly request that weaker lock level with SELECT FOR
NO KEY UPDATE.

The attached patch updated README.tuplock accordingly.

best regards,
Florian Pflug