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/create_function.sgml

@@ -257,7 +257,7 @@ CREATE [ OR REPLACE ] FUNCTION
        The name of the language that the function is implemented in.
        It can be <literal>sql</literal>, <literal>c</literal>,
        <literal>internal</literal>, or the name of a user-defined
-       procedural language, e.g. <literal>plpgsql</literal>.  Enclosing the
+       procedural language, e.g., <literal>plpgsql</literal>.  Enclosing the
        name in single quotes is deprecated and requires matching case.
       </para>
      </listitem>
@@ -431,11 +431,11 @@ CREATE [ OR REPLACE ] FUNCTION
       Functions should be labeled parallel unsafe if they modify any database
       state, or if they make changes to the transaction such as using
       sub-transactions, or if they access sequences or attempt to make
-      persistent changes to settings (e.g. <literal>setval</literal>).  They should
+      persistent changes to settings (e.g., <literal>setval</literal>).  They should
       be labeled as parallel restricted if they access temporary tables,
       client connection state, cursors, prepared statements, or miscellaneous
       backend-local state which the system cannot synchronize in parallel mode
-      (e.g.  <literal>setseed</literal> cannot be executed other than by the group
+      (e.g.,  <literal>setseed</literal> cannot be executed other than by the group
       leader because a change made by another process would not be reflected
       in the leader).  In general, if a function is labeled as being safe when
       it is restricted or unsafe, or if it is labeled as being restricted when