config.txt: pack.windowmemory limit applies per-thread

It took me a long time to notice the rider on the pack.threads
configuration option that it would multiple the memory consumption
by the number of CPUs in the machine.  Clarify that the limit
applies per-thread.

Signed-off-by: Robert de Bath <rdebath@tvisiontech.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1827,10 +1827,11 @@ pack.depth::
 	maximum depth is given on the command line. Defaults to 50.
 
 pack.windowMemory::
-	The window memory size limit used by linkgit:git-pack-objects[1]
-	when no limit is given on the command line.  The value can be
-	suffixed with "k", "m", or "g".  Defaults to 0, meaning no
-	limit.
+	The maximum size of memory that is consumed by each thread
+	in linkgit:git-pack-objects[1] for pack window memory when
+	no limit is given on the command line.  The value can be
+	suffixed with "k", "m", or "g".  When left unconfigured (or
+	set explicitly to 0), there will be no limit.
 
 pack.compression::
 	An integer -1..9, indicating the compression level for objects