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_17_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7f9c71519143b6aec71df068e022ddbe33bcc0d3

Modified Files
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doc/src/sgml/datatype.sgml | 19 ++++++++++++++++---
1 file changed, 16 insertions(+), 3 deletions(-)