mem-cache: Fix description for writeback_clean.

The description explains when we have to set this True (when a
downstream cache acts as a victim cache). Also, it describes general
(default) setup, but this seems inaccurate and not consistent with
default vaule.

Change-Id: I389adb0af0d6421e8a9672c4cf5d23510eb38242
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/62832
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/mem/cache/Cache.py

@@ -146,11 +146,14 @@ class BaseCache(ClockedObject):
     system = Param.System(Parent.any, "System we belong to")
 
     # Determine if this cache sends out writebacks for clean lines, or
-    # simply clean evicts. In cases where a downstream cache is mostly
-    # exclusive with respect to this cache (acting as a victim cache),
-    # the clean writebacks are essential for performance. In general
-    # this should be set to True for anything but the last-level
-    # cache.
+    # simply clean evicts. If this cache does not have a downstream cache,
+    # the cache should not writeback clean lines not to waste memory
+    # bandwidth. If this cache has a downstream cache whose clusivity is
+    # mostly exclusive (i.e., victim cache), this shoule be set to True.
+    # If not, there will never be any spills from read-only caches (e.g.,
+    # L1I cache, MMU cache of ARM) to the downstream cache.
+    # In case of the downstream cache is mostly inclusive, this should be
+    # set to False.
     writeback_clean = Param.Bool(False, "Writeback clean lines")
 
     # Control whether this cache should be mostly inclusive or mostly