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_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5a353a2d43eeab7554a6986b04b0a9d98ad55c92

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