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_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/7961cd49e26b35db65516847b94ee15b8b0f3580

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