Fix outdated bit in README.tuplock

Apparently this information has been outdated since first committed,
because we adopted a different implementation during development per
reviews and this detail was not updated in the README.

This has been wrong since commit 0ac5ad5134 introduced the file in
2013.  Backpatch to all live branches.

Reported-by: Will Mortensen <will@extrahop.com>
Discussion: https://postgr.es/m/CAMpnoC6yEQ=c0Rdq-J7uRedrP7Zo9UMp6VZyP23QMT68n06cvA@mail.gmail.com

src/backend/access/heap/README.tuplock

@@ -70,13 +70,8 @@ KEY SHARE        conflict
 
 When there is a single locker in a tuple, we can just store the locking info
 in the tuple itself.  We do this by storing the locker's Xid in XMAX, and
-setting infomask bits specifying the locking strength.  There is one exception
-here: since infomask space is limited, we do not provide a separate bit
-for SELECT FOR SHARE, so we have to use the extended info in a MultiXact in
-that case.  (The other cases, SELECT FOR UPDATE and SELECT FOR KEY SHARE, are
-presumably more commonly used due to being the standards-mandated locking
-mechanism, or heavily used by the RI code, so we want to provide fast paths
-for those.)
+setting infomask bits specifying the locking strength.  See "Infomask Bits"
+below for details on the bit patterns we use.
 
 MultiXacts
 ----------