pgsql: Docs: wording improvements in discussion of timestamp arithmetic

Docs: wording improvements in discussion of timestamp arithmetic.

I started out just to fix the broken markup in commit
1c2085766187031eaeaae7db4785b9e1d4241988, but got distracted by
copy-editing.  I see Bruce already fixed the markup, but I'll
commit the wordsmithing anyway.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/7489eb4d3b7fef2c2eaeb66e3bcb3fb74385b44d

Modified Files
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doc/src/sgml/func.sgml |   29 ++++++++++++++++-------------
1 file changed, 16 insertions(+), 13 deletions(-)