pgsql: Clean up grammar a bit

Clean up grammar a bit

Simplify the grammar specification of substring() and overlay() a bit,
simplify and update some comments.

Reviewed-by: Pavel Stehule <pavel.stehule@gmail.com>
Reviewed-by: Vik Fearing <vik@postgresfriends.org>
Reviewed-by: Fabien COELHO <coelho@cri.ensmp.fr>
Discussion: https://www.postgresql.org/message-id/flat/a15db31c-d0f8-8ce0-9039-578a31758adb%402ndquadrant.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/aafefb4dcbf79e8cb1439e888a9cdb3dfefa7657

Modified Files
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src/backend/parser/gram.y | 73 +++++++++++++++--------------------------------
1 file changed, 23 insertions(+), 50 deletions(-)