pgsql: doc: Some copy-editing around prefix operators

doc: Some copy-editing around prefix operators

When postfix operators where dropped in 1ed6b8956, the CREATE OPERATOR
docs were not updated to make the RIGHTARG argument mandatory in the
grammar.

While at it, make the RIGHTARG docs more concise. Also, the operator
docs were mentioning "infix" in the introduction, while using "binary"
everywhere else.

Author: Christoph Berg <myon@debian.org>
Discussion: /messages/by-id/aAtpbnQphv4LWAye@msg.df7cb.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2e640a0fa224e4233220252b360efd33c98b3e90

Modified Files
--------------
doc/src/sgml/ref/create_operator.sgml | 6 +++---
doc/src/sgml/xoper.sgml | 2 +-
2 files changed, 4 insertions(+), 4 deletions(-)