doc: core.ignoreStat clarify the --assume-unchanged effect

The assume-unchanged bit can be misunderstood. Be assertive about
the expectation that file changes should update that flag.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -366,10 +366,11 @@ proxy use, while defaulting to a common proxy for external domains.
 core.ignoreStat::
 	If true, commands which modify both the working tree and the index
 	will mark the updated paths with the "assume unchanged" bit in the
-	index. These marked files are then assumed to stay unchanged in the
-	working tree, until you mark them otherwise manually - Git will not
-	detect the file changes	by lstat() calls. This is useful on systems
-	where those are very slow, such as Microsoft Windows.
+	index. These marked files are then expected to stay unchanged in the
+	working tree. If you change them you should mark their update manually.
+	Git will normally not detect the file changes by lstat() calls.
+	This is useful on systems where those calls are very slow, such as
+	cifs/Microsoft Windows.
 	See linkgit:git-update-index[1].
 	False by default.