pgsql: Document lock taken on referenced table when adding a foreignke

Document lock taken on referenced table when adding a foreign key

This can happen for CREATE TABLE and ALTER TABLE, so a mention is added
to both of them in the concerned subsections.

Author: Adrien Nayrat
Discussion: https://postgr.es/m/c4e8af11-1dfc-766a-c953-76979b9fcdaa@anayrat.info

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3d4b143dc7fd9d060e9f11ac6c17620f58cfc33d

Modified Files
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doc/src/sgml/ref/alter_table.sgml  | 5 +++++
doc/src/sgml/ref/create_table.sgml | 4 +++-
2 files changed, 8 insertions(+), 1 deletion(-)