pgsql: doc: reformat GRAPH_TABLE examples

doc: reformat GRAPH_TABLE examples

The GRAPH_TABLE examples in ddl.sgml and queries.sgml were written
as single long lines, making their structure harder to read in the
generated documentation.

Reformat these examples so that the graph name, MATCH clause, and
COLUMNS clause appear on separate lines, making them easier to read.

Author: Koshino Taiki <koshino(at)sraoss(dot)co(dot)jp>
Reviewed-by: Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/OS9P286MB64860BD6CD6D4B4B0E1CEDE894A32@OS9P286MB6486.JPNP286.PROD.OUTLOOK.COM
Backpatch-through: 19

Branch
------
REL_19_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/07df2b253277321d68d515615ec6c0a3d70ec591

Modified Files
--------------
doc/src/sgml/ddl.sgml | 12 +++++++++---
doc/src/sgml/queries.sgml | 13 +++++++++----
2 files changed, 18 insertions(+), 7 deletions(-)