Doc: fix remaining over-length entries in SQL keywords table.

Doc: fix remaining over-length entries in SQL keywords table.

Even after the tweaking I did in commit 5545b69ae, some of the
longer keywords mentioned in the SQL standard don't fit the
available space in PDF output.

I experimented with various solutions like putting such keywords
on their own table lines, but everything looked ugly or confusing
or both; worse, the weirdness also appeared in the HTML version,
which (normally) doesn't need it.

The best answer seems to be to insert &zwsp; into long keywords
so that they can be broken into two lines when, and only when,
needed.  It doesn't look too awful if the break happens after
an underscore --- and fortunately, all the problematic keywords
have underscores.

Discussion: https://postgr.es/m/6916.1589146280@sss.pgh.pa.us