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

doc/src/sgml/datatype.sgml

@@ -3308,7 +3308,7 @@ SELECT person.name, holidays.num_weeks FROM person, holidays
        </row>
        <row>
         <entry><type>line</type></entry>
-        <entry>32 bytes</entry>
+        <entry>24 bytes</entry>
         <entry>Infinite line</entry>
         <entry>{A,B,C}</entry>
        </row>
@@ -3352,6 +3352,11 @@ SELECT person.name, holidays.num_weeks FROM person, holidays
      </tgroup>
     </table>
 
+   <para>
+    In all these types, the individual coordinates are stored as
+    <type>double precision</type> (<type>float8</type>) numbers.
+   </para>
+
    <para>
     A rich set of functions and operators is available to perform various geometric
     operations such as scaling, translation, rotation, and determining
@@ -3541,8 +3546,17 @@ SELECT person.name, holidays.num_weeks FROM person, holidays
 
     <para>
      Polygons are represented by lists of points (the vertexes of the
-     polygon). Polygons are very similar to closed paths, but are
-     stored differently and have their own set of support routines.
+     polygon). Polygons are very similar to closed paths; the essential
+     semantic difference is that a polygon is considered to include the
+     area within it, while a path is not.
+    </para>
+
+    <para>
+     An important implementation difference between polygons and
+     paths is that the stored representation of a polygon includes its
+     smallest bounding box.  This speeds up certain search operations,
+     although computing the bounding box adds overhead while constructing
+     new polygons.
     </para>
 
     <para>