Fix various typos in code and tests

Most of these are recent, and the documentation portions are new as of
v16 so there is no need for a backpatch.

Author: Justin Pryzby
Discussion: https://postgr.es/m/20230208155644.GM1653@telsasoft.com

doc/src/sgml/config.sgml

@@ -718,7 +718,7 @@ include_dir 'conf.d'
        <para>
         Determines the number of connection <quote>slots</quote> that are
         reserved for connections by roles with privileges of the
-        <link linkend="predefined-roles-table"><literal>pg_used_reserved_connections</literal></link>
+        <link linkend="predefined-roles-table"><literal>pg_use_reserved_connections</literal></link>
         role.  Whenever the number of free connection slots is greater than
         <xref linkend="guc-superuser-reserved-connections"/> but less than or
         equal to the sum of <varname>superuser_reserved_connections</varname>

doc/src/sgml/ref/create_database.sgml

@@ -89,7 +89,7 @@ CREATE DATABASE <replaceable class="parameter">name</replaceable>
         The role name of the user who will own the new database,
         or <literal>DEFAULT</literal> to use the default (namely, the
         user executing the command).  To create a database owned by another
-        role, you must must be able to <literal>SET ROLE</literal> to that
+        role, you must be able to <literal>SET ROLE</literal> to that
         role.
        </para>
       </listitem>

doc/src/sgml/ref/create_schema.sgml

@@ -89,7 +89,7 @@ CREATE SCHEMA IF NOT EXISTS AUTHORIZATION <replaceable class="parameter">role_sp
        <para>
         The role name of the user who will own the new schema.  If omitted,
         defaults to the user executing the command.  To create a schema
-        owned by another role, you must must be able to
+        owned by another role, you must be able to
         <literal>SET ROLE</literal> to that role.
        </para>
       </listitem>