doc: add float as an alias for double precision.

 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