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: alexander.kjall@hafslund.no
Author: Euler Taveira <euler@eulerto.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/175456294638.800.12038559679827947313@wrigleys.postgresql.org
Backpatch-through: 13

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>