Clarify the docstring of DfsBlockCache.reconfigure()

The docstring was copied from the local filesystem cache code, which
actually attempted to reconfigure the cache on the fly. The DFS cache is
designed to be "reconfigured" exactly once.

Change-Id: Ia0b01f5d6b6b3d3a68d65a5c229ff67c1cede5bc

org.eclipse.jgit/src/org/eclipse/jgit/storage/dfs/DfsBlockCache.java

@@ -99,9 +99,8 @@ public final class DfsBlockCache {
 	/**
 	 * Modify the configuration of the window cache.
 	 * <p>
-	 * The new configuration is applied immediately. If the new limits are
+	 * The new configuration is applied immediately, and the existing cache is
-	 * smaller than what what is currently cached, older entries will be purged
+	 * cleared.
-	 * as soon as possible to allow the cache to meet the new limit.
 	 *
 	 * @param cfg
 	 *            the new window cache configuration.