pgsql: Doc: small improvements in discussion of geometric data types.

Doc: small improvements in discussion of geometric data types.

State explicitly that the coordinates in our geometric data types are
float8.  Also explain that polygons store their bounding box.

While here, fix the table of geometric data types to show type
"line"'s size correctly: it's 24 bytes not 32.  This has somehow
escaped notice since that table was made in 1998.

Per suggestion from Sebastian Skałacki.  The size error seems
important enough to justify back-patching.

Discussion: https://postgr.es/m/172000045661.706.1822177575291548794@wrigleys.postgresql.org

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ac2ec6c57b38ea5baec08ef01d9cbabf1565017e

Modified Files
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doc/src/sgml/datatype.sgml | 20 +++++++++++++++++---
1 file changed, 17 insertions(+), 3 deletions(-)