Remove the word "virgins" for documentation

Apparently, it's no longer welcome.  Therefore replace it with "pristine",
and add some explanatory text while at it.

Reported by Brian Williams
Discussion: https://postgr.es/m/157313712259.14261.16141263269989647311@wrigleys.postgresql.org

doc/src/sgml/manage-ag.sgml

@@ -199,11 +199,12 @@ createdb -O <replaceable>rolename</replaceable> <replaceable>dbname</replaceable
    should never be changed after the database cluster has been
    initialized.  By instructing
    <command>CREATE DATABASE</command> to copy <literal>template0</literal> instead
-   of <literal>template1</literal>, you can create a <quote>virgin</quote> user
-   database that contains none of the site-local additions in
+   of <literal>template1</literal>, you can create a <quote>pristine</quote> user
+   database (one where no user-defined objects exist and where the system
+   objects have not been altered) that contains none of the site-local additions in
    <literal>template1</literal>.  This is particularly handy when restoring a
    <literal>pg_dump</literal> dump: the dump script should be restored in a
-   virgin database to ensure that one recreates the correct contents
+   pristine database to ensure that one recreates the correct contents
    of the dumped database, without conflicting with objects that
    might have been added to <literal>template1</literal> later on.
   </para>

doc/src/sgml/ref/create_database.sgml

@@ -54,8 +54,10 @@ CREATE DATABASE <replaceable class="parameter">name</replaceable>
    system database <literal>template1</literal>.  A different template can be
    specified by writing <literal>TEMPLATE
    <replaceable class="parameter">name</replaceable></literal>.  In particular,
-   by writing <literal>TEMPLATE template0</literal>, you can create a virgin
-   database containing only the standard objects predefined by your
+   by writing <literal>TEMPLATE template0</literal>, you can create a pristine
+   database (one where no user-defined objects exist and where the system
+   objects have not been altered)
+   containing only the standard objects predefined by your
    version of <productname>PostgreSQL</productname>.  This is useful
    if you wish to avoid copying
    any installation-local objects that might have been added to