doc: add float as an alias for double precision.

Although the "Floating-Point Types" section says that "float" data
type is taken to mean "double precision", this information was not
reflected in the data type table that lists all data type aliases.

Reported-by: [email protected]
Author: Euler Taveira <[email protected]>
Reviewed-by: Tom Lane <[email protected]>
Discussion: https://postgr.es/m/[email protected]
Backpatch-through: 13
(cherry picked from commit 13244ba1032573bf1a220aa4b8fb0ff395f7815a)

Author: tglsfdc

doc/src/sgml/datatype.sgml

@@ -117,7 +117,7 @@
 
       <row>
        <entry><type>double precision</type></entry>
-       <entry><type>float8</type></entry>
+       <entry><type>float</type>, <type>float8</type></entry>
        <entry>double precision floating-point number (8 bytes)</entry>
       </row>