[COMMITTERS] pgsql: Improve docs about numeric formatting patterns(to_char/to_numbe

Improve docs about numeric formatting patterns (to_char/to_number).

The explanation about "0" versus "9" format characters was confusing
and arguably wrong; the discussion of sign handling wasn't very good
either.  Notably, while it's accurate to say that "FM" strips leading
zeroes in date/time values, what it really does with numeric values
is to strip *trailing* zeroes, and then only if you wrote "9" rather
than "0".  Per gripes from Erwin Brandstetter.

Discussion: https://postgr.es/m/CAGHENJ7jgRbTn6nf48xNZ=FHgL2WQ4X8mYsUAU57f-vq8PubEw@mail.gmail.com
Discussion: https://postgr.es/m/CAGHENJ45ymd=GOCu1vwV9u7GmCR80_5tW0fP9C_gJKbruGMHvQ@mail.gmail.com

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5c81c881f7caceffb9dd3174a72a724db04d073f

Modified Files
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doc/src/sgml/func.sgml | 63 ++++++++++++++++++++++++++++++++++++--------------
1 file changed, 46 insertions(+), 17 deletions(-)