qcow2: Fix cache-clean-interval documentation

Fixing cache-clean-interval documentation following the recent change to
a default of 600 seconds on supported plarforms (only Linux currently).
Signed-off-by: NLeonid Bloch <lbloch@janustech.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

docs/qcow2-cache.txt

@@ -202,18 +202,18 @@ Reducing the memory usage
 It is possible to clean unused cache entries in order to reduce the
 memory usage during periods of low I/O activity.
 
-The parameter "cache-clean-interval" defines an interval (in seconds).
-All cache entries that haven't been accessed during that interval are
-removed from memory.
+The parameter "cache-clean-interval" defines an interval (in seconds),
+after which all the cache entries that haven't been accessed during the
+interval are removed from memory. Setting this parameter to 0 disables this
+feature.
 
-This example removes all unused cache entries every 15 minutes:
+The following example removes all unused cache entries every 15 minutes:
 
    -drive file=hd.qcow2,cache-clean-interval=900
 
-If unset, the default value for this parameter is 600. Setting it to 0
-disables this feature.
+If unset, the default value for this parameter is 600 on platforms which
+support this functionality, and is 0 (disabled) on other platforms.
 
-Note that this functionality currently relies on the MADV_DONTNEED
-argument for madvise() to actually free the memory. This is a
-Linux-specific feature, so cache-clean-interval is not supported in
-other systems.
+This functionality currently relies on the MADV_DONTNEED argument for
+madvise() to actually free the memory. This is a Linux-specific feature,
+so cache-clean-interval is not supported on other systems.

qapi/block-core.json

@@ -2895,7 +2895,8 @@
 #
 # @cache-clean-interval:  clean unused entries in the L2 and refcount
 #                         caches. The interval is in seconds. The default value
-#                         is 600, and 0 disables this feature. (since 2.5)
+#                         is 600 on supporting platforms, and 0 on other
+#                         platforms. 0 disables this feature. (since 2.5)
 #
 # @encrypt:               Image decryption options. Mandatory for
 #                         encrypted images, except when doing a metadata-only

qemu-options.hx

@@ -747,7 +747,8 @@ it which is not used for the L2 cache)
 
 @item cache-clean-interval
 Clean unused entries in the L2 and refcount caches. The interval is in seconds.
-The default value is 600. Setting it to 0 disables this feature.
+The default value is 600 on supporting platforms, and 0 on other platforms.
+Setting it to 0 disables this feature.
 
 @item pass-discard-request
 Whether discard requests to the qcow2 device should be forwarded to the data