pgsql: Doc: improve description of plpgsql's RAISE command.

Doc: improve description of plpgsql's RAISE command.

RAISE accepts either = or := in the USING clause, so fix the
syntax synopsis to show that.

Rearrange and wordsmith the descriptions of the different syntax
variants, in hopes of improving clarity.

Igor Gnatyuk, reviewed by Jian He and Laurenz Albe; minor additional
wordsmithing by me

Discussion: https://postgr.es/m/CAEu6iLvhF5sdGeat2x4_L0FvWW_SiN--ma8ya7CZd-oJoV+yqQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/56c6be57af6bd1c7eb7dff50e5f169ced4ed3045

Modified Files
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doc/src/sgml/plpgsql.sgml | 58 ++++++++++++++++++++++++++++-------------------
1 file changed, 35 insertions(+), 23 deletions(-)