Slight tweak to documentation about default lockmode

When documentation HTML is generated, there is some ambiguity about to
which section the italicised "This is the default lockmode" applies: the
text appears at the end of the LOCKMODE_EXC second and immediately
before LOCKMODE_WAIT, with no visible separators except an empty line.

The default LOCKMODE_EXC section already begins with exactly the same
statement.  This change removes the trailing redundant and
confusingly-located statement and puts the italics on the original,
unambiguously-located statement.