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