doc: add commas after 'i.e.' and 'e.g.'

This follows the American format,
https://jakubmarian.com/comma-after-i-e-and-e-g/. There is no intention
of requiring this format for future text, but making existing text
consistent every few years makes sense.

Discussion: https://postgr.es/m/20200825183619.GA22369@momjian.us

Backpatch-through: 9.5

doc/src/sgml/ref/postgres-ref.sgml

@@ -821,7 +821,7 @@ PostgreSQL documentation
 
   <para>
    To start <command>postgres</command> with a specific
-   port, e.g. 1234:
+   port, e.g., 1234:
 <screen>
 <prompt>$</prompt> <userinput>postgres -p 1234</userinput>
 </screen>