Doc: hack on table 26.1 till it fits in PDF format.

Doc: hack on table 26.1 till it fits in PDF format.

I abbreviated the heck out of the column headings, and made a few
small wording changes, to get it to build warning-free.  I can't
say that the result is pretty, but it's probably better than
removing this table entirely.

As of this commit, we have zero "exceed the available area" warnings
in a US-letter PDF build, and one such warning (about an 863-millipoint
overrun) in an A4 build.  I expect to get rid of that one by renaming
wait events, so I'm not doing anything about it at the formatting
level.

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