Doc: reword text explaining the --maintenance-db option

The previous text was a little clumsy.  Here we improve that.

Author: David Rowley <dgrowleyml@gmail.com>
Reported-by: Noboru Saito <noborusai@gmail.com>
Reviewed-by: David G. Johnston <david.g.johnston@gmail.com>
Discussion: https://postgr.es/m/CAAM3qnJtv5YbjpwDfVOYN2gZ9zGSLFM1UGJgptSXmwfifOZJFQ@mail.gmail.com
Backpatch-through: 13

doc/src/sgml/ref/clusterdb.sgml

@@ -248,9 +248,8 @@ PostgreSQL documentation
       <term><option>--maintenance-db=<replaceable class="parameter">dbname</replaceable></option></term>
       <listitem>
        <para>
-        Specifies the name of the database to connect to to discover which
-        databases should be clustered,
-        when <option>-a</option>/<option>--all</option> is used.
+        When the <option>-a</option>/<option>--all</option> is used, connect
+        to this database to gather the list of databases to cluster.
         If not specified, the <literal>postgres</literal> database will be used,
         or if that does not exist, <literal>template1</literal> will be used.
         This can be a <link linkend="libpq-connstring">connection

doc/src/sgml/ref/reindexdb.sgml

@@ -352,9 +352,8 @@ PostgreSQL documentation
       <term><option>--maintenance-db=<replaceable class="parameter">dbname</replaceable></option></term>
       <listitem>
        <para>
-        Specifies the name of the database to connect to to discover which
-        databases should be reindexed,
-        when <option>-a</option>/<option>--all</option> is used.
+        When the <option>-a</option>/<option>--all</option> is used, connect
+        to this database to gather the list of databases to reindex.
         If not specified, the <literal>postgres</literal> database will be used,
         or if that does not exist, <literal>template1</literal> will be used.
         This can be a <link linkend="libpq-connstring">connection

doc/src/sgml/ref/vacuumdb.sgml

@@ -544,9 +544,8 @@ PostgreSQL documentation
       <term><option>--maintenance-db=<replaceable class="parameter">dbname</replaceable></option></term>
       <listitem>
        <para>
-        Specifies the name of the database to connect to to discover which
-        databases should be vacuumed,
-        when <option>-a</option>/<option>--all</option> is used.
+        When the <option>-a</option>/<option>--all</option> is used, connect
+        to this database to gather the list of databases to vacuum.
         If not specified, the <literal>postgres</literal> database will be used,
         or if that does not exist, <literal>template1</literal> will be used.
         This can be a <link linkend="libpq-connstring">connection