doc: suggest Bash for large-fd script

* doc/tar.texi (Multi-Volume Archives): Suggest Bash for shell
script that might use >&10.  Problem reported by Edward F Eaglehouse in
<http://lists.gnu.org/archive/html/bug-tar/2012-02/msg00002.html>.

doc/tar.texi

@@ -11435,7 +11435,10 @@ archive being created (as given by @option{--file} option) and
 
 @smallexample
 @group
-#! /bin/sh
+#! /bin/bash
+# For this script it's advisable to use a shell, such as Bash,
+# that supports a TAR_FD value greater than 9.
+
 echo Preparing volume $TAR_VOLUME of $TAR_ARCHIVE.
 
 name=`expr $TAR_ARCHIVE : '\(.*\)-.*'`