Allow the wal_buffers setting to be auto-tuned to a reasonable value.

If wal_buffers is initially set to -1 (which is now the default), it's
replaced by 1/32nd of shared_buffers, with a minimum of 8 (the old default)
and a maximum of the XLOG segment size.  The allowed range for manual
settings is still from 4 up to whatever will fit in shared memory.

Greg Smith, with implementation correction by me.

doc/src/sgml/config.sgml

@@ -1638,12 +1638,25 @@ SET ENABLE_SEQSCAN TO OFF;
       </indexterm>
       <listitem>
        <para>
-        The amount of memory used in shared memory for WAL data.  The
-        default is 64 kilobytes (<literal>64kB</>).  The setting need only
-        be large enough to hold the amount of WAL data generated by one
-        typical transaction, since the data is written out to disk at
-        every transaction commit.  This parameter can only be set at server
-        start.
+        The amount of shared memory used for WAL data that has not yet been
+        written to disk.  The default setting of -1 selects a size equal to
+        1/32nd (about 3%) of <xref linkend="guc-shared-buffers">, but not less
+        than <literal>64kB</literal> nor more than the size of one WAL
+        segment, typically <literal>16MB</literal>.  This value can be set
+        manually if the automatic choice is too large or too small,
+        but any positive value less than <literal>32kB</literal> will be
+        treated as <literal>32kB</literal>.
+        This parameter can only be set at server start.
+       </para>
+
+       <para>
+        The contents of the WAL buffers are written out to disk at every
+        transaction commit, so extremely large values are unlikely to
+        provide a significant benefit.  However, setting this value to at
+        least a few megabytes can improve write performance on a busy
+        server where many clients are committing at once.  The auto-tuning
+        selected by the default setting of -1 should give reasonable
+        results in most cases.
        </para>
 
        <para>