Correct docs for the default locale_provider of a new database.

If the locale provider is not specified, it defaults to be the same as
the template from which it was created. Previously, the documentation
said the default was libc.

Also adjust wording of CREATE DATABASE and CREATE COLLATION docs to be
definite that there are exactly two possible collation providers.

Discussion: https://postgr.es/m/6befdaada61c046b67f3b269f7fa6f069a35803e.camel%40j-davis.com
Reviewed-by: Nathan Bossart

doc/src/sgml/ref/create_collation.sgml

@@ -119,11 +119,11 @@ CREATE COLLATION [ IF NOT EXISTS ] <replaceable>name</replaceable> FROM <replace
      <listitem>
       <para>
        Specifies the provider to use for locale services associated with this
-       collation.  Possible values
-       are: <literal>icu</literal>,<indexterm><primary>ICU</primary></indexterm>
-       <literal>libc</literal>.
-       <literal>libc</literal> is the default.
-       The available choices depend on the operating system and build options.
+       collation.  Possible values are
+       <literal>icu</literal><indexterm><primary>ICU</primary></indexterm>
+       (if the server was built with ICU support) or <literal>libc</literal>.
+       <literal>libc</literal> is the default.  See <xref
+       linkend="locale-providers"/> for details.
       </para>
      </listitem>
     </varlistentry>

doc/src/sgml/ref/create_database.sgml

@@ -198,10 +198,12 @@ CREATE DATABASE <replaceable class="parameter">name</replaceable>
       <listitem>
        <para>
         Specifies the provider to use for the default collation in this
-        database.  Possible values are:
-        <literal>icu</literal>,<indexterm><primary>ICU</primary></indexterm>
-        <literal>libc</literal>.  <literal>libc</literal> is the default.  The
-        available choices depend on the operating system and build options.
+        database.  Possible values are
+        <literal>icu</literal><indexterm><primary>ICU</primary></indexterm>
+        (if the server was built with ICU support) or <literal>libc</literal>.
+        By default, the provider is the same as that of the <xref
+        linkend="create-database-template"/>. See <xref
+        linkend="locale-providers"/> for details.
        </para>
       </listitem>
      </varlistentry>