Rather than intermixing the discussion of text-string and binary-string
functions, make a clean break, moving all discussion of binary-string
operations into section 9.5.  This involves some duplication of
function descriptions between 9.4 and 9.5, but it seems cleaner on the
whole since the individual descriptions are clearer (and on the other
side of the coin, it gets rid of some duplicated descriptions, too).

Move the convert*/encode/decode functions to a separate table, because
they don't quite seem to fit under the heading of "binary string
functions".

Also provide full documentation of the textual formats supported by
encode() and decode() (which was the original goal of this patch
series, many moons ago).

Also move the table of built-in encoding conversions out of section 9.4,
where it no longer had any relevance whatsoever, and put it into section
23.3 about character sets.  I chose to put both that and table 23.2
(multibyte-translation-table) into a new <sect2> so as not to break up
the flow of discussion in 23.3.3.

Also do a bunch of minor copy-editing on the function descriptions
in 9.4 and 9.5.

Karl Pinc, reviewed by Fabien Coelho, further hacking by me

Discussion: https://postgr.es/m/20190304163347.7bca4897@slate.meme.com