Doc: fix misleading syntax synopses for targetlists.

Doc: fix misleading syntax synopses for targetlists.

In the syntax synopses for SELECT, INSERT, UPDATE, etc,
SELECT ... and RETURNING ... targetlists were missing { ... }
braces around an OR (|) operator.  That allows misinterpretation
which could lead to confusion.

David G. Johnston, per gripe from [email protected].

Discussion: https://postgr.es/m/172193970148.915373.2403176471224676074@wrigleys.postgresql.org