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.
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