s390/debug: Update description of resize operation

With commit 1204777 ("s390/debug: keep debug data on resize")
the behavior of a debug area resize operation was changed. Update the
associated documentation to reflect this change.

Fixes: 1204777 ("s390/debug: keep debug data on resize")
Reported-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

Author: oberpar

Documentation/arch/s390/s390dbf.rst

@@ -243,9 +243,8 @@ Examples:
 
 Changing the size of debug areas
 ------------------------------------
-It is possible the change the size of debug areas through piping
-the number of pages to the debugfs file "pages". The resize request will
-also flush the debug areas.
+To resize a debug area, write the desired page count to the "pages" file.
+Existing data is preserved if it fits; otherwise, oldest entries are dropped.
 
 Example: