pgsql: Improve description of pg_attrdef in documentation

Improve description of pg_attrdef in documentation

The reference to pg_attribute is switched to a link, which is more
useful for the html documentation.  The conditions under which a default
value is defined for a given column are made more general.

Author: Daniel Gustafsson
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/0E8748E3-8B7D-445E-9ABA-09DA5C7345CC@yesql.se

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0993b8ada53395a8c8a59401a7b4cfb501f6aaef

Modified Files
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doc/src/sgml/catalogs.sgml | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)