Fix recovery_prefetch docs.

Correct a typo and a couple of sentences that weren't updated to reflect
recent changes to the code.

Reported-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/20220407125555.GC24419%40telsasoft.com

doc/src/sgml/config.sgml

@@ -3685,8 +3685,9 @@ include_dir 'conf.d'
        <para>
         Whether to try to prefetch blocks that are referenced in the WAL that
         are not yet in the buffer pool, during recovery.  Valid values are
-        <literal>off</literal> (the default), <literal>on</literal> and
-        <literal>try</literal>.  The setting <literal>try</literal> enables
+        <literal>off</literal>, <literal>on</literal> and
+        <literal>try</literal> (the default).  The setting
+        <literal>try</literal> enables
         prefetching only if the operating system provides the
         <function>posix_fadvise</function> function, which is currently used
         to implement prefetching.  Note that some operating systems provide the

doc/src/sgml/monitoring.sgml

@@ -3049,11 +3049,9 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
 
   <para>
    The <structname>pg_stat_recovery_prefetch</structname> view will contain
-   only one row.  It is filled with nulls if recovery has not run or
-   <xref linkend="guc-recovery-prefetch"/> is not enabled.  The
-   columns <structfield>wal_distance</structfield>,
-   <structfield>block_distance</structfield>
-   and <structfield>io_depth</structfield> show current values, and the
+   only one row.  The columns <structfield>wal_distance</structfield>,
+   <structfield>block_distance</structfield> and
+   <structfield>io_depth</structfield> show current values, and the
    other columns show cumulative counters that can be reset
    with the <function>pg_stat_reset_shared</function> function.
   </para>

doc/src/sgml/wal.sgml

@@ -812,7 +812,7 @@
    The <xref linkend="guc-maintenance-io-concurrency"/> and
    <xref linkend="guc-wal-decode-buffer-size"/> settings limit prefetching
    concurrency and distance, respectively.  By default, it is set to
-   <literal>try</literal>, which enabled the feature on systems where
+   <literal>try</literal>, which enables the feature on systems where
    <function>posix_fadvise</function> is available.
   </para>
  </sect1>