pg_xlogdump: document --path behavior

The previous --path documentation and --help output were wrong in both
its meaning and the defaults.

Reviewed-by: Michael Paquier

Backpatch-through: 9.6

doc/src/sgml/ref/pg_xlogdump.sgml

@@ -117,9 +117,12 @@ PostgreSQL documentation
       <term><option>--path=<replaceable>path</replaceable></option></term>
       <listitem>
        <para>
-        Directory in which to find log segment files.  The default is to search
-        for them in the <literal>pg_wal</literal> subdirectory of the current
-        directory.
+        Specifies a directory to search for log segment files or a
+        directory with a <literal>pg_wal</literal> subdirectory that
+        contains such files.  The default is to search in the current
+        directory, the <literal>pg_wal</literal> subdirectory of the
+        current directory, and the <literal>pg_wal</literal> subdirectory
+        of <envar>PGDATA</envar>.
        </para>
       </listitem>
      </varlistentry>

src/bin/pg_xlogdump/pg_xlogdump.c

@@ -679,8 +679,9 @@ usage(void)
 	printf(_("  -e, --end=RECPTR       stop reading at log position RECPTR\n"));
 	printf(_("  -f, --follow           keep retrying after reaching end of WAL\n"));
 	printf(_("  -n, --limit=N          number of records to display\n"));
-	printf(_("  -p, --path=PATH        directory in which to find log segment files\n"
-			 "                         (default: ./pg_wal)\n"));
+	printf(_("  -p, --path=PATH        directory in which to find log segment files or a\n"
+			 "                         directory with a ./pg_wal that contains such files\n"
+			 "                         (default: current directory, ./pg_wal, PGDATA/pg_wal)\n"));
 	printf(_("  -r, --rmgr=RMGR        only show records generated by resource manager RMGR\n"
 			 "                         use --rmgr=list to list valid resource manager names\n"));
 	printf(_("  -s, --start=RECPTR     start reading at log position RECPTR\n"));