doc: Add doc about osd scrub {during recovery|chunk {min|max}| sleep}

doc/rados/configuration/osd-config-ref.rst

@@ -237,6 +237,17 @@ scrubbing operations.
 :Type: Integer in the range of 0 to 24
 :Default: ``24``
 
+
+``osd scrub during recovery``
+
+:Description: Allow scrub during recovery. Setting this to ``false`` will disable
+              scheduling new scrub (and deep--scrub) while there is active recovery.
+              Already running scrubs will be continued. This might be useful to reduce
+              load on busy clusters.
+:Type: Boolean
+:Default: ``true``
+
+
 ``osd scrub thread timeout`` 
 
 :Description: The maximum time in seconds before timing out a scrub thread.
@@ -281,6 +292,32 @@ scrubbing operations.
 :Default: Once per week. ``7*60*60*24``
 
 
+``osd scrub chunk min``
+
+:Description: The minimal number of object store chunks to scrub during single operation.
+              Ceph blocks writes to single chunk during scrub.
+
+:Type: 32-bit Integer
+:Default: 5
+
+
+``osd scrub chunk max``
+
+:Description: The maximum number of object store chunks to scrub during single operation.
+
+:Type: 32-bit Integer
+:Default: 25
+
+
+``osd scrub sleep``
+
+:Description: Time to sleep before scrubbing next group of chunks. Increasing this value will slow
+              down whole scrub operation while client operations will be less impacted.
+
+:Type: Float
+:Default: 0
+
+
 ``osd deep scrub interval``
 
 :Description: The interval for "deep" scrubbing (fully reading all data). The