Doc: Review for "Optionally prefetch referenced data in recovery."

Typos, corrections and language improvements in the docs, and a few in
code comments too.

Reported-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/20210409033703.GP6592%40telsasoft.com

doc/src/sgml/config.sgml

@@ -3621,7 +3621,7 @@ include_dir 'conf.d'
         pool after that.  However, on file systems with a block size larger
         than
         <productname>PostgreSQL</productname>'s, prefetching can avoid a
-        costly read-before-write when a blocks are later written.
+        costly read-before-write when blocks are later written.
         The default is off.
        </para>
       </listitem>

doc/src/sgml/wal.sgml

@@ -816,9 +816,7 @@
    prefetching mechanism is most likely to be effective on systems
    with <varname>full_page_writes</varname> set to
    <varname>off</varname> (where that is safe), and where the working
-   set is larger than RAM.  By default, prefetching in recovery is enabled
-   on operating systems that have <function>posix_fadvise</function>
-   support.
+   set is larger than RAM.  By default, prefetching in recovery is disabled.
   </para>
  </sect1>
 

src/backend/access/transam/xlogprefetch.c

@@ -31,12 +31,14 @@
  * stall; this is counted with "skip_fpw".
  *
  * The only way we currently have to know that an I/O initiated with
- * PrefetchSharedBuffer() has that recovery will eventually call ReadBuffer(),
- * and perform a synchronous read.  Therefore, we track the number of
+ * PrefetchSharedBuffer() has completed is to wait for the corresponding call
+ * to XLogReadBufferInRedo() to return.  Therefore, we track the number of
  * potentially in-flight I/Os by using a circular buffer of LSNs.  When it's
- * full, we have to wait for recovery to replay records so that the queue
- * depth can be reduced, before we can do any more prefetching.  Ideally, this
- * keeps us the right distance ahead to respect maintenance_io_concurrency.
+ * full, we have to wait for recovery to replay enough records to remove some
+ * LSNs, and only then can we initiate more prefetching.  Ideally, this keeps
+ * us just the right distance ahead to respect maintenance_io_concurrency,
+ * though in practice it errs on the side of being too conservative because
+ * many I/Os complete sooner than we know.
  *
  *-------------------------------------------------------------------------
  */

src/backend/utils/misc/guc.c

@@ -2774,7 +2774,7 @@ static struct config_int ConfigureNamesInt[] =
 	{
 		{"wal_decode_buffer_size", PGC_POSTMASTER, WAL_ARCHIVE_RECOVERY,
 			gettext_noop("Maximum buffer size for reading ahead in the WAL during recovery."),
-			gettext_noop("This controls the maximum distance we can read ahead n the WAL to prefetch referenced blocks."),
+			gettext_noop("This controls the maximum distance we can read ahead in the WAL to prefetch referenced blocks."),
 			GUC_UNIT_BYTE
 		},
 		&wal_decode_buffer_size,