pgsql: doc: for various substring funcs, document if only first match

doc: for various substring funcs, document if only first match

Reported-by: troy@frericks.us

Discussion: /messages/by-id/162614304115.701.2392941350859387646@wrigleys.postgresql.org

Backpatch-through: 13

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d629fcf4b36ca8c1e4e9677745ff7d725433ee19

Modified Files
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doc/src/sgml/func.sgml | 35 ++++++++++++++++++++---------------
1 file changed, 20 insertions(+), 15 deletions(-)