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: /messages/by-id/172000045661.706.1822177575291548794@wrigleys.postgresql.org

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/b261649c0a860cd49e11f3d52d3f117a10fde2b4

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