pgsql: ecpg: Reject multiple header items in GET/SET DESCRIPTOR

ecpg: Reject multiple header items in GET/SET DESCRIPTOR

Previously, ecpg accepted multiple descriptor header items in GET DESCRIPTOR
and SET DESCRIPTOR, but generated broken C code when they were used.
Although the grammar allowed this syntax, the implementation did not actually
support it.

This commit tightens the ecpg grammar so the header form of GET/SET DESCRIPTOR
accepts only a single header item, matching the implementation and preventing
generation of broken C code.

Also update the documentation synopsis accordingly.

Backpatch to all supported versions.

Author: Masashi Kamura <kamura(dot)masashi(at)fujitsu(dot)com>
Reviewed-by: Hayato Kuroda <kuroda(dot)hayato(at)fujitsu(dot)com>
Reviewed-by: Lakshmi G <lakshmigcdac(at)gmail(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/OS9PR01MB13174AD7D1829D0644B6BB90E9447A@OS9PR01MB13174.jpnprd01.prod.outlook.com
Backpatch-through: 14

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9e8fd9f7ab56965dfe012d6be1df152ea8f62aec

Modified Files
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doc/src/sgml/ecpg.sgml | 6 +++---
src/interfaces/ecpg/preproc/ecpg.trailer | 12 ++----------
2 files changed, 5 insertions(+), 13 deletions(-)