Doc: introduce and document "&zwsp;" for allowing optional line breaks.

We already had a couple of places using zero-width spaces for formatting
hackery, and we're going to need more if we ever want the PDF manuals to
look decent.  But please let's not write hard-coded Unicode escapes.
We can avoid that by using a custom entity, which also provides a place
to put a teeny bit of documentation about what it is and how to use it.

I'd previously posted a patch using "&break;" for this, but on reflection
that would be horrible to grep for.  Instead let's use "&zwsp;", based
on the name of the Unicode symbol ("zero width space").

Discussion: https://postgr.es/m/9326.1581457869@sss.pgh.pa.us

doc/src/sgml/postgres.sgml

@@ -11,6 +11,13 @@
 
 <!ENTITY reference  SYSTEM "reference.sgml">
 
+<!--
+Zero-width space.  Use this to allow line breaks at desirable places in
+table cells, examples, etc. without causing an unwanted space when the
+break is not needed in a wider output rendering.
+-->
+<!ENTITY zwsp "&#x200B;">
+
 ]>
 
 <book id="postgres">