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

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/88d934f0387a66ba372643913f99e845d0ea144a

Modified Files
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doc/src/sgml/func.sgml     | 10 +++++-----
doc/src/sgml/postgres.sgml |  7 +++++++
2 files changed, 12 insertions(+), 5 deletions(-)